Thyroid allostasis in drug-free affective disorder patients

To assess the thyroid allostasis in drug-free patients with affective disorder.

Patients with major depressive disorder or bipolar disorder as drug-free, defined as those without psychiatric drugs exposure for at least 4 months before admission, from a tertiary hospital were recruited in this cross-sectional study. The primary outcomes were "structure parameters of thyroid homeostasis", which include "thyroid's secretory capacity" (SPINA-GT), "sum step-up activity of deiodinases" (SPINA-GD), the ratio of total to free thyroxine and "thyroid homeostasis central set point" (TSH index and "thyroid feedback quantile-based index" [TFQI]), calculated by TSH and thyroid hormones measured at admission. A healthy population and non-affective psychiatric disorder (schizophrenia) from the same catchment area were recruited as two comparison groups.

A total of 1263 cases of major depressive disorder, 1619 cases of bipolar disorder, 1186 cases of schizophrenia, and 162 healthy controls were included in the study. Compared to healthy control, GD and ratio of total to free thyroxine were lower in affective disorders. Bipolar with mania episode had higher GT than bipolar with depressive episode and major depressive disorder (median level at 3.70 vs. 3.04 and 3.03, respectively). Compared with healthy control, schizophrenia had higher TSH index and TFQI, but no increase in these parameters in major depressive disorder and bipolar disorder.

Affective disorders have a unique profile of thyroid allostasis with impaired step-up deiodinase activity and reduced serum protein binding of thyroid hormones, but no change in thyroid homeostasis central set point. Mania episode may be associated with higher thyroid secretory capacity.