Sher L, Rosenthal NE, Wehr TA.
Section on Biological Rhythms, National Institute of Mental Health, Bethesda, MD 20892-1390, USA.
J Affect Disord. 1999 Dec;56(2-3):195-9.

Seasonal affective disorder (SAD) is characterized by recurrent episodes of depression in the fall and winter that alternate with nondepressed periods in the spring and summer. Because some symptoms of SAD, such as decreased energy and weight gain, also occur in hypothyroidism, it is possible that individuals with SAD have a subtle decrease in thyroid function. To test this hypothesis, we studied blood levels of free thyroxine (T4) and thyroid-stimulating hormone (TSH) in SAD patients and matched controls in the winter.

We found that free T4 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. Future research will be needed to determine whether the difference in thyroid function between SAD patients and controls is an epiphenomenon or is related to the biological mechanisms that cause symptoms of SAD.