Propylthiouracil (PTU) Pharmacology in the Rat, II. Effects of PTU on Thyroid Function*

DAVID S. COOPER,t J. DAVID KIEFFER, RACHELLE HALPERN, VELIA SAXE, HEIDI MOVER, FARAHE MALOOF, AND E. CHESTER RIDGWAY Thyroid Unit and Department of Medicine, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts 02114

ABSTRACT. Using a sensitive and specific RIA for propyl- in both groups. thiouracil (PTU), we examined the effects of short term (1 week) After PTU withdrawal, thyroid PBI recovered with a ti/2 of and long term (1 month) PTU treatment on thyroid function in 1.09 days after 1 week on PTU, but recovery was prolonged (ti/2 the rat, and correlated changes in thyroid function with serum = 2.8 days) after 1 month of treatment. Log thyroid PTU and and thyroid PTU levels. After 1 week, dose-dependent decreases log thyroid PBI were linearly related after PTU withdrawal (r in thyroid PBI, serum T4, and serum T3 were observed, with = 0.97; P < 0.001) after 1 week but not after 1 month. Serum T4 concomitant elevations in the serum rT3 to T4 ratio and serum and serum T3 remained below control values for 2 days, but then TSH. Fifty percent suppression of thyroid PBI occurred at a rapidly normalized, with T3 values rising transiently above the PTU concentration in the drinking water of 0.0005% (ED50), control value. This rebound occurred at a time when PTU was with concomitant serum and thyroid PTU levels of 0.3 jtg/ml still present within the thyroid, before thyroid PBI had returned and 300 ng/thyroid, respectively. After 1 month of PTU, serum to baseline. T4 values were lower than after 1 week of treatment for all PTU These data indicate a close inverse relationship between PTU concentrations, but values for the other thyroid functional var- dose and both thyroid hormone biosynthesis and peripheral T4 iables were similar to those in the 1 week group at comparable deiodination. In addition, short and long term PTU treatments PTU dosage. The PTU dose-response curve for thyroid PBI was have quantitatively similar effects on thyroid function, although similar to that seen after 1 week of treatment, with an ED50 of recovery of thyroid function is prolonged after long term treat- 0.0004%. ment. The biexponential disappearance of PTU from the serum After discontinuation of PTU treatment, PTU disappeared is compatible with a two-compartment model of PTU distribu- from serum in a biexponential fashion, with an early rapid tion. The early increase in thyroid PTU after drug withdrawal distribution phase (ti/2 = ~4 h) and a second slower elimination is suggestive of an inhibitory effect of PTU upon its own uptake phase (ti/2 = ~2.6 days). In the thyroid, an initial increase in by the thyroid, whereas the faster disappearance of PTU from PTU content was seen up to 18 h after PTU withdrawal; the thyroid than from serum is consistent with intrathyroid drug thereafter, thyroid PTU declined linearly, with a ti/2 of 1.4 days metabolism. {Endocrinology 113: 921, 1983)

HE in vivo pharmacology of the antithyroid drug of thyroid function. In addition, thyroid recovery pat- Tpropylthiouracil (PTU) in the rat has been studied terns after discontinuation of short and long term regi- by a large number of investigators (1-17). Most phar- mens have been examined. In a companion study (18), macokinetic studies, however, have involved either single we have described in detail the development and valida- injections of radiolabeled PTU or short treatment tion of the PTU RIA used for the present studies, and courses (<1 week) with unlabeled PTU. Although im- have provided data concerning serum and thyroid PTU portant information has been gained from these studies, levels after 1 week and 1 month of treatment. they provide limited insight into the effects of long term PTU treatment on thyroid function. The present inves- Materials and Methods tigations were conducted to explore more fully the effects Outbred male CD rats (Charles River Breeding Laboratories, of different durations of PTU exposure on various indices Wilmington, MA), weighing 200-250 g, were used for all studies. The conditions of animal maintenance were similar to those Received January 4,1983. reported previously (19). The rats were fed a regular diet Address requests for reprints to: Dr. David S. Cooper, Thyroid Unit, containing 1.7 ppm iodide (Purina rodent laboratory chow no. Massachusetts General Hospital, Boston, Massachusetts 02114. 5001) and water containing PTU in varying concentrations. * A preliminary account of parts of this research was presented as Abstract T-5 at the 57th Meeting of the American Thyroid Association, Groups of approximately 50 rats were used for the following Minneapolis, MN, 1981. This work was supported by USPHS Grants studies. AM-16791 and AM-07028. t Recipient of a New Investigator Award from the NIH (1-R23-AM- Exp I. This experiment was designed to assess serum and 28465-01). thyroid PTU levels and PTU effects on thyroid function after

921 922 COOPER ET AL. Endo • 1983 Volll3«No3

1 week of PTU treatment. Groups of seven or eight animals PBI or in serum concentrations of T4, T3, rT3, or TSH. each were given PTU in their drinking water in concentrations Thereafter, significant and progressive decreases in thy- ranging from 0.0001-0.01%, and were autopsied 1 week later. roid PBI and serum T4 and T3 were noted with increasing Exp II. This experiment was similar to Exp I, except that the PTU concentration. Increases in serum TSH into the duration of treatment was 1 month instead of 1 week. range seen after thyroidectomy (Kieffer, J. D., unpub- lished observations) were noted, and a dose-dependent Exp III. This experiment was designed to study PTU disap- elevation of the rT3 to T4 ratio was seen. [Since serum pearance rates and thyroid recovery patterns after short and T4 is the primary source of circulating rT3 (24), the rT3 long term PTU exposure. Animals were maintained on a con- to T ratio was employed to correct rT data for the stant concentration of PTU (0.05%) for either 1 week or 1 4 3 month. The drug was withdrawn, and groups of six or seven decreasing serum T4 concentrations.] rats were autopsied at intervals from 4 h to 14 days after drug The changes in thyroid PBI and serum T4 were plotted discontinuation. against the log PTU concentration in the drinking water, For all experiments, animals were injected with 40-60 /iCi the log PTU serum concentration, and the thyroid PTU 123I 1 h before autopsy. Animals were bled by decapitation content (Fig. 1). Fifty percent suppression of thyroid PBI without anesthesia, and serum was separated by centrifugation. occurred at a PTU concentration of 0.0005% correspond- The thyroid glands were rapidly dissected, weighed, and indi- ing to serum and thyroid PTU concentrations of 0.3 ng/ vidually homogenized in glass homogenizers in 1.5 ml ice-cold ml (1.8 HM) and 300 ng/gland (88 fiM), respectively. Fifty 0.1 M NaCl in 0.05 M Na K phosphate buffer, pH 7.4. The percent suppression of T4 required higher serum and thyroid homogenates were spun at 2300 x g for 45 min, and thyroid PTU levels (~1 txg/va\ and 500 ng/gland, respec- the supernatants were saved for the determination of the intra- tively). Maximal changes in PBI and T4 occurred over a thyroid PTU concentration. Serum and thyroid extracts were less than 10-fold range of thyroid PTU (50-500 ng/ stored at -20 C until assay. gland), but over a 100-fold range of serum PTU (0.05- Determination of thyroid PBI. To determine intrathyroid PBI, 5.0 400 (A 0.1% BSA in 0.05 M phosphate buffer were added to a 100-yu.l aliquot of thyroid homogenate. The protein was precip- Exp II itated by the addition of 500 n\ 10% trichloroacetic acid (TCA). The mixture was spun at 2300 X g for 10 min, and the pellet PTU levels and changes in thyroid function with in- was washed once with 500 (A 5% TCA. Intrathyroid PBI was creasing PTU concentration in the drinking water after calculated as the ratio of TCA-precipitable radioactivity to total 1 month of treatment are shown in Table 2. Compared radioactivity. with the 1 week treatment group, T4 values were lower at each PTU concentration, although the differences Assays. PTU, T4, T3, rT3, and TSH were measured by RIA, as were significant only at PTU concentrations of 0.0005% previously described (18-21). As noted before (18), the PTU RIA that was employed only measured free or unbound PTU and 0.005%. In contrast, thyroid PBI, serum T3 and within the thyroid. Therefore, thyroid PTU refers only to this TSH, and the rT3 to T4 ratio after 1 month of treatment fraction (~75%) of the total thyroid PTU. did not differ significantly from those seen at the same PTU concentrations after 1 week of treatment. Statistical methods. Experimental values were expressed as the Changes in thyroid PBI and serum T4 after 1 month mean ± SE. Molar thyroid PTU concentrations were calculated of treatment were plotted against log PTU concentration from thyroid drug content and thyroid weight (nanograms per in the drinking water, log serum PTU concentration, and g = nanograms per ml). The estimated amount of PTU ingested thyroid PTU content (Fig. 2). The PTU concentration was based on an average water intake of 20 ml/day • rat. The required for 50% suppression of thyroid PBI was approx- correlation and linearity of data were assessed with the method of least squares. Comparisons limited to two groups were eval- imately 0.0004%, similar to that seen in the 1 week uated by Student's t test. Multiple comparisons between the treatment group. The serum and thyroid PTU concen- means of control and treatment groups were made by Tukey's trations required for 50% PBI suppression after 1 month wholly significant difference procedure (22). Corrections for of treatment (~0.4 jig/ml and 250 ng/gland, respectively) heterogeneity of variance were made as described by Snedecor were also similar to those seen after 1 week of treatment and Cochran (23). (Fig. 1, b and c). In the 1 week group, higher concentrations of PTU were required for 50% suppression of T4 Results than for 50% suppression of PBI (see above). This disparity between PBI and T4 was not observed in the 1 Exp I month group. Serum PTU levels and changes in thyroid function Exp III with increasing PTU dose are presented in Table 1. At the lowest PTU concentration in the drinking water For this experiment, animals were treated with 0.05% (0.0001%), there were no significant changes in thyroid PTU for 1 week or 1 month. The drug was then with- PTU PHARMACOLOGY IN THE RAT 923

TABLE 1. Thyroid functional indexes and PTU levels after 1 week of treatment

Intrathyroid Serum T4 Serum T3 Serum rT3:T4 Serum TSH Serum PTU (fig/ Thyroid PTU Dose of PTU (%) 3 PBI (%) dl) (ng/dl) x 10 (ng/ml) ml) (Mg/gland) Control (n = 7) 89 ± 1 4.2 ± 0.3 60 ± 2 1.9 ± 0.1 2.9 ± 0.5 0.0001 (n = 8) 85 ± 2 5.8 ± 0.4 54 ± 5 1.4 ± 0.1 4.6 ± 1.0 0.046 ± 0.006 0.062 ± 0.009 0.0005 (n = 7) 42 ± 3° 3.8 ± 0.4 48 ± 3 2.6 ± 0.3 11.7 ± 1.5° 0.32 ± 0.02 0.313 ± 0.059 0.001 (n = 7) 24 ± 6° 2.8 ± 0.4 43 ± 4* 4.1 ± 0.5* 9.8 ± 0.7 0.53 ± 0.12 0.376 ± 0.044 0.005 (n = 7) 8.0 ± 1.5° 1.6 ± 0.2° 36 ± 2° 3.9 ± 0.5" 11.8 ± 1.8" 3.00 ± 0.34 0.502 ± 0.086 0.01 (n = 7) 3.0 ± 0.3° 1.0 ± 0.1" 34 ± 3° 6.2 ± 0.5° 10.4 ± 0.6° 5.73 ± 0.50 0.543 ± 0.062 0 P < 0.01 vs. control. 6 P < 0.05 vs. control.

100 80 80

20 20 0

II -20 -20 -40 -40 .00001 .0001 .001 .01 .1 .oi 0.1 1.0 10 0.2 0.4 0.6 0.8 PTU (% in drinking water) Serum PTU (jig/ml) Intrathyroidal PTU (jjg/thyroid)

FIG. 1. Changes in thyroid PBI and serum T4 after 1 week of PTU treatment us. PTU concentration (a), serum PTU (b), and thyroid PTU (c). The EDso was higher for T4 (0.0025%) than for thyroid PBI (0.0005%). Changes in thyroid function occurred over a narrow range of thyroid PTU levels (10-fold) and over a broad range of serum PTU levels (100-fold). The negative value for the percent decrease in serum T4 refers to the increase in serum T4 at 0.0001% PTU (see Table 1).

TABLE 2. Thyroid functional indexes and PTU levels after 1 month of treatment Intrathyroid Serum T (ng/ Serum T Serum rT :T Serum TSH Serum PTU (fig/ Thyroid PTU Dose of PTU (%) 4 3 3 4 PBI(%) dl) (ng/dl) X103 (ng/ml) ml) gland) Control (n = 7) 86 ± 2 4.5 ± 0.4 65 ± 3 1.8 ± 0.2 1.4 ± 0.3 0.0001 (n = 8) 80 ± 2 4.5 ± 0.5 67 ± 5 1.5 ± 0.2 3.3 ± 0.8 0.073 ± 0.004° 0.098 ± 0.011* 0.0005 (n = 7) 35 ± 4C 1.4 ± 0.2"-c 59 + 7 5.7 ± 0.7" 11.3 ± 0.5c 0.57 ± 0.11 0.309 ± 0.032 0.001 (n = 7) 14 ± 2C 2.2 ± 0.4d 73 ± 5 2.9 ± 0.3d 9.7 ± 0.6c 0.71 ± 0.11 0.306 ± 0.042 0.005 (n = 7) 3.5 ± 0.3c 0.6 ± 0.03c 39 ± 6 2.7 ± 0.6 13.4 ± 1.2C 5.4 ± 0.23° 0.957 ± 0.087° 0.01 (n = 7) 2.9 ± 0.2c 0.7 ± 0.046>c 33 ± 3C 4.1 ± 0.5c 13.1 ± 0.9c 13.7 ± 0.94° 0.606 ± 0.041 " P < 0.01 vs. 1 week group. 6 P < 0.02 vs. 1 week group. 0 P < 0.01 vs. control. dP< 0.05 us. control.

IS 100 80 60 40 20 0 3; .ooooi .0001 .001 .01 100 0 PTU (% in drinking water) Serum PTU (ug/ml) Intrathyroidal PTU (/ig/thyroid)

FIG. 2. Changes in thyroid PBI and serum T4 after 1 month of PTU treatment vs. PTU concentration (a), serum PTU (b), and thyroid PTU (c). Serum T4 and thyroid PBI had similar ED50 values. 924 COOPER ET AL. Endo • 1983 Volll3«No3 drawn, and PTU levels and thyroid function were assessed serially. 1.0 1.0

Disappearance of PTU from serum and thyroid gland The disappearance of PTU from the serum after 1 o.i week of treatment is depicted in Fig. 3a. Analysis by the method of residuals (25) resolved the disappearance into two phases: 1) a rapidly declining phase (ti/2 = 4.1 h), .01 and 2) a second, slower disappearance phase (ti/2 = 2.67 .01 days). A very similar pattern of disappearance from serum was observed after 1 month of PTU treatment, with ti/2 values of 4.4 h and 2.55 days for the rapid and slow phases, respectively (Fig. 3b). I 10 8 10 Figure 4a displays the data for the disappearance of Time (days) PTU from the thyroid gland after 1 week of treatment. FIG. 4. Disappearance of PTU from the thyroid gland after 1 week (a) During the first 18 h after discontinuation of the drug, and 1 month (b) of PTU treatment. Note the early increase in thyroid mean thyroid PTU levels were higher than those deter- PTU over the first 24 h in both groups. *, P < 0.05 compared with the mined immediately after drug withdrawal; the difference PTU level before PTU discontinuation. For 1 week group, ti/2 = 1.4 was statistically significant (P < 0.05) at 12 h. The days, r = 0.92, P < 0.001; for 1 month group, tU2 = 1.2 days, r = 0.91, subsequent disappearance of PTU from the thyroid con- P < 0.001. Error bars are within data points. stituted a single phase, with a ti/2 of 1.4 days. A similar pattern of an initial rise, followed by a single phase 100 disappearance (ti/2 = 1.2 days), was observed in rats treated with PTU for 1 month (Fig. 4b); in this case, the early increase in the thyroid PTU content approached, but did not attain, statistical significance compared with 10 levels before PTU discontinuation.

2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 Time (days) FIG. 5. Recovery of thyroid PBI following withdrawal of PTU after 1 week (a) and 1 month (b) of PTU treatment. The ty2 of recovery was 1.09 days after 1 week (r = 0.96; P < 0.001) and 2.8 days after 1 month (r = 0.98; P < 0.001). Error bars are within data points.

Recovery of thyroid function Recovery of thyroid PBI is shown in Fig. 5. After 1 week of PTU treatment, PBI returned toward the control level, with a half-time of 1.09 days. During the period of drug disappearance from the thyroid, the extent of PBI inhibition conformed closely to a power function of the thyroid PTU content; this can be inferred from the linearity of the log-log plot (Fig. 6). After 1 month of 8 treatment, in contrast, the half-time of recovery of thy- Time (days) roid PBI was 2.8 days, and there was a much weaker FIG. 3. Disappearance of PTU from the serum following its discontin- relationship between log thyroid PTU and log percent uation after 1 week (a) and 1 month (b) of treatment. Two phases of inhibition of thyroid PBI (r = 0.51; P = NS). disappearance are evident, a) Rapid phase, ti/2 = 4.1 h, r = 0.86, P < Data for the recovery of serum T4 and T3 are shown 0.001; slow phase, ti/2 = 2.67 days, r = 0.91, P < 0.001. b) Rapid phase, tV2 = 4.4 h, r = 0.99, P < 0.001; slow phase, t1/2 = 2.6 days, r = 0.99, P in Figs. 7 and 8. After 1 week of PTU treatment, serum < 0.001. Error bars are within data points. T4 levels remained significantly suppressed for 48 h after PTU PHARMACOLOGY IN THE RAT 925

lOO

S 50

I. 0 2 4 6 8 icp14 Time (days)

FlG. 8. Recovery of serum T3 after discontinuation of PTU treatment. The shaded area represents the mean ± SD for 14 control animals. *, P .01 .05 .5 < 0.05 vs. control; **, P < 0.01 us. control. Intrathyroidal PTU (jag/thyroid) at a time when PTU was still present within the gland FlG. 6. Log thyroid PBI us. log thyroid PTU after discontinuation of and thyroid PBI had not yet returned to the control the week of PTU treatment (r = 0.97; P < 0.001). value.

Discussion As would be anticipated, inverse relationships were observed between the PTU concentration in the drinking water and all thyroid function indexes (thyroid PBI and serum T4 and T3). For thyroid PBI, 50% inhibitory PTU concentrations were attained at a PTU concentration of 0.0005% (serum PTU, 0.3-0.6 Mg/ml), corresponding to a PTU intake of about 125 ^g/day (0.625 mg kg"1-day). Previous studies in humans suggested that serum PTU levels of 3 Mg/ml or more are necessary for 50% inhibition of thyroid iodide organification (21) and the serum free 2 4 6 8 10 14 T3 index (26). Since PTU doses of 2-3 mg/kg are needed to achieve these serum levels in man (21), the present Time (days) data suggest that PTU is approximately 3- to 5-fold more FlG. 7. Recovery of serum T4 after discontinuation of PTU treatment. potent in the rat than in man. These findings are in The shaded area represents the mean ± SD for 14 control animals. *, P < 0.05 vs. control; **, P < 0.01 vs. control. general agreement with the studies of Stanley and Ast- wood (27, 28), who reported that PTU was 10-fold more the discontinuation of the drug. At times after 48 h, the potent in the rat than in man. One potential source of T4 levels of PTU-treated rats did not differ significantly this disparity is a species difference in PTU binding by from those of controls. A very similar pattern of T4 serum proteins (29). suppression and recovery was observed after 1 month of Thyroid PTU levels of about 300 ng/gland (88 nM) PTU treatment. were required for 50% inhibition of thyroid iodide organ- Serum T3 followed a course somewhat different from ification. This concurs with the acute PTU treatment that of T4. In both the 1-week and the 1-month studies, data of Nakashima et al. (12), who found that greater serum T3 was significantly depressed for less than 2 days than 95% inhibition of thyroid iodide organification was after discontinuation of PTU treatment, rose to and obtained with PTU doses of 1.5 ^mol or more (255 /ig), briefly above normal, and then remained in the normal producing thyroid PTU concentrations of 40 ixM. In the range. Comparison of Figs. 4 and 5 with Fig. 8 reveals present study, a somewhat higher thyroid PTU concen- that serum T3 levels were clearly normal or supranormal tration (170 /AM) was required for full (>95%) inhibition 926 COOPER ET AL. Endo • 1983 Volll3«No3 of thyroid PBI. It is also of interest to compare the disappearance phase, with a half-time of 2.6 days, was present in vivo thyroid PTU data to those previously also observed in both treatment groups. These data sug- obtained in vitro. Taurog (30) and Davidson et al. (31) gest that PTU clearance from the serum is not altered found that the iodination of thyroglobulin in vitro was by long term (1-month) vs. short term (1-week) hypothy- 50% inhibited by PTU concentrations of 18.5 and 29 /xM, roidism. Recently, Giles et al. (17) presented data show- respectively. ing that PTU clearance was slightly lower than the After 1 week of PTU treatment, 50% suppression of control value in rats with PTU-induced hypothyroidism. thyroid PBI occurred at lower serum and thyroid PTU The two studies are not strictly comparable, however, levels than were required for 50% suppression of serum since the methodology for determining PTU clearance T4 levels. In contrast, after 1 month, the dose-response in the report of Giles et al. (17) was different from that curves for thyroid PBI and serum T4 were similar. This used in the present study. discrepancy may be due to the fact that stored thyroid The biexponential disappearance of PTU from the T4 was not depleted after 1 week of PTU treatment and, serum suggests a two-compartment model for this drug. therefore, was able to make a contribution to serum T4 In such models, the serum and heavily perfused tissues regardless of PTU administration. At 1 month, however, constitute the central compartment, whereas poorly per- very little stored thyroid T4 was likely to be present. It fused tissues comprise the peripheral compartment (25). is possible that a treatment period even longer than 1 Moreover, the findings that biological responses to PTU month might have produced effects different from the were linearly related to intrathyroid PTU levels over ones observed in the present study. However, the data of most of the response range is consistent with a two- Griessen and Lemarchand-Beraud (10) show that the compartment model, in which the site of drug action (the effects of PTU on plasma PBI and thyroid iodine are thyroid) is in the peripheral compartment. PTU disap- maximal within 17 days and remain unchanged after up peared from the thyroid with a ty2 that was shorter than to 6 months of continuous PTU treatment. that for the slower phase of disappearance from serum At PTU concentrations of 0.005% or more, thyroid (~1.3 days vs. 2.7 days). This suggests that, in contrast PBI was fully inhibited, serum T4 fell to 38% of the basal to the simplest two-compartment model, drug elimina- value after 1 week and to 12% of the control value after tion takes place in the thyroid as well as in the central 1 month of treatment. In contrast, serum T3 declined compartment. The hypothesis of drug elimination in the less markedly, to 57-61% of the control value in both thyroid is in accord with the suggestion of Taurog (30) treatment groups. A similar discordant pattern between that PTU disappearance from the thyroid is more likely T4 and T3 was observed by Mannisto et al. (15) in rats to involve intrathyroid drug metabolism than transfer treated with 0.01% PTU for 9 days. The reason for the back into the circulation. differing T3 and T4 responses may be that TSH prefer- Despite rapidly falling serum PTU concentrations over entially stimulates thyroid T3 secretion (32). In addition, the first 24 h after drug discontinuation, the thyroid there is evidence that even 1 week of PTU treatment gland continued to accumulate PTU. After 1 week, sig- results in intrathyroid iodine deficiency (10), which also nificantly higher thyroid PTU was noted 12 h after favors thyroid production of T3 over T4 (32). Finally, a cessation of treatment than during the treatment period. direct effect of PTU (in small doses) to cause preferential A similar trend was noted in the 1 month group of synthesis of T3 over T4 has been described (2, 33). animals as well. These data suggest that the plateau of It can be seen from Figs. 1 and 2 that the curves thyroid PTU levels with increasing PTU dose observed relating inhibition of thyroid PBI to log PTU dose, log by us (15) and in other studies (14) may be related in serum PTU, and thyroid PTU were sigmoidal. The slopes part to inhibition by PTU of PTU entry into the thyroid. of the curves are steeper for thyroid PTU than for serum Marchant et al. (6) and Nakashima et al. (12) made PTU, suggesting that alterations in thyroid function are similar observations based on decreasing PTU uptake by more sensitive to small changes in thyroid drug levels the thyroid with increasing single doses of drug. Thus, than to equivalent changes in serum drug concentrations. large doses of PTU may actually provide no additional The disappearance of PTU from serum and thyroid therapeutic benefit than smaller doses. A similar sugges- and the concomitant recovery of thyroid function were tion was made by Nakashima et al. (12) based on their monitored after 1 week and 1 month of PTU treatment. single dose experiments. When PTU was discontinued, serum PTU initially de- The recovery of thyroid iodide organification after 1 clined rapidly, with an early half-time of disappearance week of PTU treatment paralleled the disappearance of of 4 h, regardless of treatment duration. This half-life is drug from the thyroid gland, rather than from the serum. similar to the distribution phase obtained in the rat by This suggests that thyroid PTU is the major determinant Sitar and Thornhill (7) and Aungst et al. (14) after single of this aspect of thyroid function in PTU-treated ani- injections of radiolabeled PTU. A second, slower serum mals. However, since thyroid PTU is clearly related to PTU PHARMACOLOGY IN THE RAT 927

PTU dose and PTU serum levels, thyroid function is 6. Marchant B, Alexander WD, Robertson JWK, Lazarus JH 1971 Concentration of 35S-propylthiouracil by the thyroid gland and its obviously dependent on these variables as well. The relationship to anion trapping mechanism. Metabolism 20:989 relationship between thyroid recovery and thyroid PTU 7. Sitar DS, Thornhill DP 1972 Propylthiouracil: absorption, metab- disappearance was much poorer after 1 month of PTU olism and excretion in the albino rat. J Pharmacol Exp Ther 183:440 treatment. Indeed, the recovery period was twice as long 8. Papapetrou PD, Marchant B, Gavras H, Alexander WD 1972 after 1 month of treatment as after 1 week despite similar Biliary excretion of 35S-labelled propylthiouracil, methimazole and PTU disappearance curves. This suggests that chronic carbimazole in untreated and pentobarbitone pretreated rats. Biochem Pharmacol 21:363 drug therapy may have secondary as well as primary 9. Marchant B, Alexander WD, Lazarus JH, Lees J, Clark DH 1972 effects. For example, thyroid iodide deficiency is more The accumulation of 36S-antithyroid drugs by the thyroid gland. J severe after 1 month of PTU treatment (10) than after Clin Endocrinol Metab 34:847 10. Griessen M, Lemarchand-Beraud T 1973 Thyrotropin secretion 1 week, and in vitro studies have shown that the effects and metabolism in rats during propylthiouracil treatment. Endo- of PTU on thyroid peroxidase are enhanced in the pres- crinology 92:166 ence of low iodide concentrations (30). Furthermore, 11. Lindsay RH, Hill JB, Kelly K, Vaughn A 1974 Excretion of propylthiouracil and its metabolites in rat bile and urine. Endocri- thyroid PTU metabolism is less rapid in iodine-deficient nology 94:1689 animals (30). 12. Nakashima T, Taurog A, Riesco G 1978 Mechanism of action of After discontinuation of PTU in both the 1 week and thioureylene antithyroid drugs: factors affecting intrathyroidal metabolism of propylthiouracil and methimazole in rats. Endocrinol- 1 month groups, serum T3 levels rose to supranormal ogy 103:2187 values at 4-5 days, while serum T4 was not significantly 13. Lees JFH, Alexander WD 1978 Effects of chronic treatment of greater than control at any time after PTU discontin- intact and hypophysectomized rats with thyroid-stimulating hormone on the metabolism of [35S]methimazole and [35S]propyl- uation. This rebound of thyroid function after with- thiouracil. Endocrinology 103:1394 drawal of antithyroid drugs has been noted by others and 14. Aungst BJ, Vesell ES, Shapiro JR 1979 Unusual characteristics of studied in detail by Studer and Greer (32). In accord the dose-dependent uptake of propylthiouracil by thyroid gland in vivo. Biochem Pharmacol 28:1479 with the present results, these workers found that the T3 15. Mannisto PT, Ranta T, Leppaluoto J 1979 Effects of methylmer- content of thyroid extracts rose to 500% of control by 4 captoimidazole (MMI), propylthiouracil (PTU), potassium per- days after PTU withdrawal in animals maintained for 7 chlorate (KCIO4) and potassium iodide (KI) on the serum concentrations of thyrotrophin (TSH) and thyroid hormones in the rat. days on a low iodine diet and PTU (0.15%). Acta Endocrinol (Copenh) 91:271 The present observations complement these data and 16. Lang JCT, Marchant B, Alexander WD 1980 Comparison of anti- suggest that the rebound phase of T , and possibly T , thyroid drug metabolism in the rat and guinea pig. Pharmacology 3 4 20:32 biosynthesis occurs at a time after drug withdrawal when 17. Giles HG, Long JP, Orrego H, Sellers EM 1982 Mechanism of thyroid PBI has not yet recovered fully and when thyroid alterations in propylthiouracil disposition after long-term therapy. PTU levels are still easily detectable. This paradox can Clin Pharmacol Ther 31:559 be partially resolved by considering that a thyroid PTU 18. Halpern R, Cooper DS, Kieffer JD, Saxe V, Mover H, Maloof F, Ridgway EC 1983 Propylthiouracil (PTU) pharmacology in the level of 300 ng/gland or more is required for significant rat. I. Serum and thyroid PTU measurements by radioimmunoas- antithyroid effects, whereas values much lower than say. Endocrinology 113:915 these were seen within 1-2 days after drug withdrawal. 19. Kieffer JD, Mover H, Federico P, Maloof F 1976 Pituitary-thyroid axis in neonatal and adult rats: comparison of the sexes. Endocri- nology 98:295 Acknowledgment 20. Gautvik KM, Tashjian Jr AH, Kourides IA, Weintraub BD, Grae- ber CT, Maloof F, Suzuki K, Zuckerman JE 1974 Thyrotropin- The expert secretarial assistance of Ms. Sharon Melanson is greatly releasing hormone is not the sole physiologic mediator of prolactin appreciated. release during suckling. N Engl J Med 290:1162 21. Cooper DS, Saxe VC, Meskell M, Maloof F, Ridgway EC 1982 Acute effects of propylthiouracil (PTU) on thyroidal iodide organ- References ification and peripheral iodothyronine deiodination: correlation with serum PTU levels measured by radioimmunoassay. J Clin 1. Astwood EB, Bissel A, Hughes AM 1945 Further studies on the Endocrinol Metab 54:101 chemical nature of compounds which inhibit the function of the 22. Dunnett CW1970 Multiple comparisons. In: McArthur JW, Colton thyroid gland. Endocrinology 37:456 T (eds) Statistics in Endocrinology. MIT Press, Cambridge, p 84 2. Slingerland DW, Graham DE, Josephs RK, Mulvey PF Jr, Trakas 23. 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