European Review for Medical and Pharmacological Sciences 2009; 13: 341-349 Thyroid function tests: a review G. SHIVARAJ, B. DESAI PRAKASH, V. SONAL, K. SHRUTHI, H. VINAYAK, M. AVINASH Department of Biochemistry, J. N. Medical College, Belgaum 590010, Karnataka (India) Abstract. – In this paper, we review the combination can give comprehensive data that tests that are executed to aid the diagnosis of would enhance the diagnostic accuracy1. thyroid dysfunction. Thyroid function tests pro- vide information at physiological, pathological and anatomical levels. Along with history and Serum Total Thyroxine (T4/TT4) physical examination they owe to many specific The concentration of total T4 in adults ranges findings that are associated with thyroid func- from 5 to 12 µg/dL (64 to 154 nmol/L)1. The con- tioning. So an attempt has been made to put for- centrations of T4 below or above this range in ab- ward a gist of thyroid function tests. Serum tests sence of thyroid dysfunction, is as a result of an ab- of thyroid function are serum total thyroxine normal level of serum Thyroid Binding Globulin (T4), serum total triiodothyronin (T3), free thyrox- ine (FT4), free triiodothyronin (FT3), reverse tri- (TBG). Such abnormally high values are observed iodothyronin (rT3), thyroid stimulating hormone in many physiological conditions in women with (TSH), serum calcitonin and protein thyroglobu- hyperestrogenic state of pregnancy1. Hyperthy- lin (Tg). The serological tests are antithyroglobu- roidism and hypothyroidism can be associated with lin antibodies (ATA) and antimicrosomal antibod- abnormal menstrual cycles2. Reference ranges for ies (AMA). An invasive test for histologic exami- thyroid function tests for TT in cord-blood is 7.4- nation is done by fine needle aspiration cytology 4 (FNAC) and noninvasive test includes ultra- 13.1 µg/dL, 1-2 weeks is 9.9-16.6 µg/dL, 1-4 sonography, magnetic resonance imaging, and months is 7.8-16.5 µg/dL, 1-5 years is 7.3-15 positron emission tomography. Further molecu- µg/dL, and 5-10 years is 6.4-13.3 µg/dL3. Small lar study provides molecular markers for thyroid seasonal variations and changes related to high alti- cancer. These tests can provide greater sensitiv- tude, cold and heat are also seen. The variation is ity and specificity that enhance the likelihood of also related to postural changes in serum proteins early detection of ambiguous thyroid disease concentration and true circadian variation. There is with only minimal clinical findings. Lastly, in vivo tests are thyroidal radioiodine and iodide uptake increased binding to serum proteins in cases of Fa- is also done. milial Dysalbuminemic Hyperthyroxinemia (FDH) which shows increased TBG1. Subclinical primary Key Words: hypothyroidism is more common in persons with 4 Serum total thyroxine, Free thyroxine, Thyroid stim- chronic kidney disease (CKD) . ulating hormone, Serum thyroglobulin, Serum calci- In thyrotoxic state serum TT4 concentration is tonin. elevated and said to be hyperthyroidism that can be caused by Graves’ disease, Plummer’s disease (toxic thyroid adenoma), early phase of acute thyroiditis, thyrotoxic factitia, struma ovarii and normal in some cases of Luft’s syndrome (Hy- permetabolic Mitochondrial Miopathy). Introduction In hypothyroidism serum TT4 concentration is low in case of thyroid gland failure. It can be fur- Thyroid functions have subtle clinical features ther classified into primary, secondary and tertiary. associated with some forms of thyroid dysfunc- The cause for primary hypothyroidism can be tion. The clinicians must decide which test is best gland destruction and severe inborn error of hor- suiting to diagnose or exclude disorder. It is em- monogenesis, secondary hypothyroidism is caused phasized that single thyroid function test (TFT) is by pituitary failure and tertiary by hypothalamic not absolute in diagnostic accuracy and it must be failure. Sometimes it can be subclinical if there is thus a careful selection of such tests so that their thyroid transporter defect or deiodinase defect1. Corresponding Author: Shivaraj Gowda, MD; e-mail: [email protected] 341 G. Shivaraj, B. Desai Prakash, V. Sonal, K. Shruthi, H. Vinayak, M. Avinash Serum Total Triiodothyronine (T3/TT3) are rarely used as stand-alone tests, but are em- A normal serum TT3 concentration in adult ployed in conjunction with a binding protein esti- range from 80-190 ng/dL1. It reflects the func- mate test i.e. Thyroid Hormone Binding Ratio tional state of peripheral tissue rather than secre- (THBR) to form a Free Hormone Index i.e. FT4I 12 tory performance of the thyroid gland. Sex dif- or FT3I . ference is small, but age difference is more dra- matic. The decline of mean TT4 is also observed Free Thyroxine (FT4) in old age all though not in healthy subjects, The normal values for FT4 in adults range 1 which suggest that fall in TT3, might reflect from 1.0 to 3.0 ng/dL (13 to 39 pmol/L) .A prevalence of non-thyroidal illness rather than minute amount of thyroid hormone circulates in an effect of age alone2. Positive co-relation be- the blood in a free form, not bound to serum pro- tween serum TT3 level and body weight has teins. It is in reversible equilibrium with the been observed5. Hormones are iodothyronines bound hormone and represents the diffusible that control growth and development, as well as fraction of the hormone capable of traversing cel- brain function and metabolism. The T3 and T4 lular membranes to exert its effects on body tis- level were found to be significantly raised in the sues. Although changes in serum hormone-bind- moderate depression as compared to the healthy ing proteins affect both the total hormone con- controls6. centration and the corresponding fraction circu- Thyroid study on mutations in the monocar- lating free in the euthyroid person, the absolute boxylate cell membrane transporter 8 (MCT8) concentration of free hormone remains constant genes, located on the X chromosome (Xq13-q21 and correlates with the tissue hormone level and 13 and Xq12-q13) has established the physiological its biologic effect . Serum FT4 may be sup- importance of MCT8 as a thyroid hormone trans- pressed in the patients with thyroidal illness and porter. This syndrome combines thyroid and neu- transiently rise in acute thyroidal illness, when rological abnormalities. MCT8 gene (also known thyroid-binding protein frequently falls11. as SLC16A2 and XPCT) defect should be sus- pected in front of psychomotor impairment (se- Free Triiodothyronine (FT3) vere developmental delay, truncal hypotomia and The normal adult reference value is 0.25-0.65 1 limb spasticity) and high serum T3,low T4 and ng/dL (3.8-10 nmol/L) . Free triiodothyronine rT3 concentrations. However, the neurological (FT3) measures the very tiny amount of T3 that manifestations of this syndrome cannot be ex- circulates unbound. It is useful in looking for hy- plained by the thyroid function tests. The pheno- perthyroidism or thyroxine overplacement in type is different from that of global hormone de- women who are pregnant or taking any effective 11 ficiency or excess. Treatment with L-T4 (physio- drugs that varies the TBG like estrogen . More logical doses) has not corrected in several pa- consistently, patients with a variety of non-thy- 1 tients the phenotype. It has been recommended roidal illnesses have low FT3 levels . This de- the use of higher doses of L-T4 during pregnancy. crease is characteristic of all conditions associat- MCT8 knockout mices have demonstrated tissue- ed with depressed serum TT3 concentrations due specific TH excess and deprivation due to differ- to a diminished conversion of T4 to T3 in periph- ent tissue dependency on MCT8 for cellular thy- eral tissues14. 7-8 roid hormone uptake . Marked elevations in both FT4 and FT3 con- The principle uses for obtaining the serum T3 centrations in the absence of hypermetabolism are to determine the severity of hyperthyroidism, are typical of patients with resistance to thyroid 15 and to confirm the diagnosis of suspected thyro- hormone . The FT3 concentration is usually nor- toxicosis in which serum T4 levels are normal or mal or even high in hypothyroid persons living in equivocal9. In addition it may be required to carry areas of severe endemic iodine deficiency and 16 out the test in cases of functioning thyroid adeno- their FT4 levels are, however, normal or low . In- mas, where T3 toxicosis may be present and such formation concerning this value can be the most patients may have normal or borderline elevated important parameter in evaluation of thyroid serum T4 levels along with suppressed serum function because it relates to patients status al- 10 TSH levels .Serum T3 is misleadingly elevated though other mechanisms exists for cell to con- in women who are pregnant or who take oral es- trol the active amount of the thyroid hormone by trogen, due to the high serum levels of TBG in autoregulation of receptor17 and regulation of 11 18 these conditions . TT4 and TT3 measurements deiodinase activity . Rarely, a defect in thyroid 342 Thyroid function tests: a review hormone transport in the cells would abolish the rT3 concentration in serum reflects both tissue 7 free hormone and metabolic effect co-relation . supply and metabolism of T4 and identify condi- The free hormone concentration is high in thyro- tions that favor this particular pathway of T4 toxicosis, low in hypothyroidism,and normal in degradation. The normal range in adult serum for 19 1 euthyroidism . rT3 is 14-30 ng/dl (0.22-0.46 nmol/L) although varying values have been reported. It is elevated Triiodothyronine Resin Uptake Test (T3RU) in subjects with high TBG and in some individu- Values correlate inversely with the concentra- als with Familial Dysalbuminemic Hyperthyrox- 1 2 tion of unsaturated TBG . A high resin uptake is inemia (FDH) .Serum rT3 levels are normal in seen with hyperthyroidism and with chronic liver hypothyroid patients treated with T4, indicating disease, nephrotic syndrome, anabolic steroid ad- that peripheral T4 metabolism is an important 20 ministration, and high dose corticosteroid admin- source of circulating rT3 .