Free thyroxine and thyroid-stimulating hormone levels in individuals with seasonal affective disorder

Leo Sher, M.D.

Our research work, “Free thyroxine and thyroid-stimulating hormone levels in patients with seasonal affective disorder and matched controls” was published 25 years ago in the December 1999 issue of the Journal of Affective Disorders (1).

Some symptoms of seasonal affective disorder (SAD), such as decreased energy and weight gain, also occur in hypothyroidism. It is possible that people with SAD have a subtle decrease in thyroid function. To test this hypothesis, we studied blood levels of free thyroxine and thyroid-stimulating hormone (TSH) in SAD patients and matched controls in the winter.

Patients with SAD and controls matched for gender, age, and menstrual status participated in the study. Healthy volunteers were required to have very low seasonality: a Global Seasonality Score (GSS) of less than 6. The study was conducted in the winter, from December through February.

We found that free thyroxine blood levels were slightly but significantly lower in patients than in healthy volunteers. The difference between TSH levels in SAD patients and controls was not statistically significant. Abnormal thyroid values would have excluded patients from the study. The current findings might thus have been attenuated as a result of our screening practices.

A small decrease in daytime free thyroxine blood levels in SAD patients could arise from a mild ‘euthyroid sick’ syndrome which is often seen in patients with depressive disorders. A small decrease in free thyroxine blood levels in SAD patients vs. controls in the daytime might also be the consequence of diminished stimulation of the thyroid gland owing to a reduction in the nocturnal rise of TSH secretion.

Reference

1. Sher L, Rosenthal NE, Wehr TA. Free thyroxine and thyroid-stimulating hormone levels in patients with seasonal affective disorder and matched controls. J Affect Disord. 1999 Dec;56(2-3):195-9. doi: 10.1016/s0165-0327(99)00049-x.