J Ayub Med Coll Abbottabad 2006; 18(4) EDITORIAL DERANGED THYROID HORMONE STATUS IN NON-THYROID ILLNESSES; SICK EUTHYROID SYNDROME Tariq Saeed Mufti, Asif Jielani* Head of Surgical Division, Ayub Teaching Hospital, Abbottabad, *Consultant Nuclear Physician, Nuclear Medicine Department, Institu te of Nuclear Medicine, Oncology & Radiotherapy, Abbottabad Critical illness is characterized by multiple and (rT3), leading to the general term low T3 syndrome. complex metabolic, immunological and endocrine Thyroid-stimulating hormone (TSH), thyroxine (T4), alterations1. Abnormalities in thyroid homeostasis also free T4, and free T4 index (FTI) are also affected in occur in variety of non-thyroid illnesses. Changes in variable degrees based on the severity and duration of thyroid hormone metabolism in critical illnesses the NTI. As the severity of the NTI increases, both appear to reflect a continuum which relates primarily serum T3 and T4 levels may drop and gradually to the severity of the underlying disorders2,3. The normalize as the patient recovers. prevalence of one or more abnormalities of thyroid Serum TSH alterations in euthyroid patients function tests in patients with non thyroidal medical with non thyroidal illnesses include transiently illnesses has been reported from 40% to 70%4,5. The reduced or elevated basal TSH values, blunted TSH condition is reported in starvation6, sepsis 7, surgery8, response to TRH, diminished or absent diurnal myocardial infarction9, CABG surgery10, bone marrow rhythms of TSH, and altered TSH glycosylation and transplantation11, and, in fact, probably any severe bioactivity19,20. Slightly decreased serum TSH has illness. been documented in elderly patients20, in healthy Girvent et al12 noted that such changes are centenarians22. Food may also affect TSH secretion23. highly prevalent in elderly patients with acute surgical TSH levels might be considered as a sensitive marker problems, and is associated with poor nutrition and of a lack of thyroid hormone since the concentrations higher sympathetic response. The general hormonal of TSH sharply increase in primary hypothyroidism response to critical illness involves activation of the even before serum T4 and T3 fall below the normal pituitary-adrenal axis, inhibition of the pituitary- reference range (so called sub-clinical thyroid & pituitary- gonadal axes13. These normal hypothyroidism)24. In NTIS, however, despite the responses distort standard reference intervals. In case decrease in serum T3 (and T4 in severe cases), the of the pituitary-thyroid axis, evaluation is further concentrations of TSH typically remain within low to complicated by changes in nutrition and major effects normal range25.Conversely, there is a blunted response of medication. Evidence suggests that these patients of TSH to thyrotropin-releasing hormone (TRH), and may not really be euthyroid, especially at the tissue low TSH levels are associated with poor prognosis26. level14. Taken together, these findings suggest that a major Based upon the fact that patients with systemic change in thyroid hormone set point regulation occurs illness are clinically euthyroid, Wartofsky and in NTI. Accordingly, prolonged critically ill patients Bunnan15 in 1982 used the term sick euthyroid show diminished TSH pulsatility, characterized by an syndrome to describe spectrum of thyroid absent nocturnal TSH surge and decreased TSH pulse abnormalities associated with non thyroidal illness. amplitude27. On occasion, transient TSH elevation Euthyroid Sick Syndrome (ESS) and Non thyroidal occurs while the patient is still ill. The illness syndrome (NITS) are terms used alternatively pathophysiology of this apparent thyroid gland in the literature16,17. resistance to TSH is not clear28. The interpretational difficulty due to NTIS Levels of T3 rapidly decrease during leading to mismanagement of co-existing goiter, a starvation e.g. post operative period or early in the course pathology of sizable incidence in certain geographical of a critical illness. Low serum total-T3 level has been distribution including Pakistan, is a significant recognized in more than 70% of hospitalized patients possibility. with non-thyroidal illness29. Starvation, and more Initial data of thyroid function tests from precisely carbohydrate deprivation, appears to rapidly Institute of Nuclear Medicine, Oncology and inhibit deiodination of T4 to T3 by Type 1 Radiotherapy (INOR), Pakistan is indicative of this iodothyronine-deiodinase in the liver, thus inhibiting incidence where 51 out of 648 tests on patients with generation of T3, and preventing metabolism of goiter showed abnormalities of T3, T4, & TSH which reverse T3, resulting in low T3 and high reverse T3 were un-interpretable18. concentration30. The serum concentration of reverse T3 The most prominent alterations are low is increased in non-thyroidal illness, except in patients with serum triiodothyronine (T3) and elevated reverse T3 renal failure and HIV infection32. Alteration in reverse T3 1 J Ayub Med Coll Abbottabad 2006; 18(4) metabolism appear to be disease specific. Both free and non-thyroid illness is challenging. As a practical total reverse T3 levels increase as a result of reduced matter, the changes in patients with non thyroidal clearance of reverse T3, however, production rate of rT3 illness must be distinguished from those resulting remains normal. Reduced metabolic clearance is from thyroid disease, which is often rightly suspected predominantly due to decreased activity of the type I in patients with other illnesses. Clinical evaluation of iodothyronine 59-monodeiodinase (5`-MDI) in the signs and symptoms of hypothyroidism may be tissues); 5`-MDI de-iodinates T4 to T3 and rT3 to 3, extremely difficult, to discern in a patient in the ICU 39-diiodothyronine (T2)31. Thus, serum reverse T3 who typically has multiple medical problems and may levels do not reliably differentiate patients with be receiving medication for sedation. Inter current euthyroid sick patients, and are not clinically useful32. complications such as infections, may further Increased turnover of T3 and T4 in the hyper complicate the difficult interpretation of thyroid metabolic phase of illness may also contribute to low function tests. serum and tissue T3 concentrations2. Total T3, free Changes in TSH should be assessed in T3 levels and T3 daily production rate are decreased patients with NTI subjects using a sensitive third- in non-thyroid illness33 while Total T4, free T4 and generation assay 47. A normal serum TSH most likely daily production rate of T4 is normal in low T3 excludes primary thyrotoxicosis or hypothyroidism syndrome45. and suggests that the patient is euthyroid Suppressed Although the isolated low T3 state usually TSH levels may be seen in small percentage of represents the mildest form of non-thyroidal illness, the critically ill patients (eg, those receiving dopamine or magnitude of the drop in T3 level reflects the severity glucocorticoids). Elevated TSH levels may also occur 25 of illness. A very low serum T3 level has been in NTI upon recovery ; however, these values rarely associated with an increased mortality rate in patients exceed 10 mU/L28. with hepatic cirrhosis, congestive heart failure, and It is prudent not to rely solely on thyroid 35 other systemic dis eases . function tests in the setting of NTI, and a combination Serum total T4 levels can be decreased (ie, low T4 of tests should be considered in separating primary syndrome) typically inpatients with more chronic and hypothyroid from euthyroid patients due to NTI 36-38 severe systemic illness . Majority of patients have In conclusion while interpreting thyroid serum freeT4 either being normal or slightly function tests the existence of NITS/ ESS may be kept 39 decreased, but occasionally elevated . This variability in mind in order to have more appropriate in free-T4 level reflects both the assay method used management of patient. and the underlying illness. 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