Lack of seasonal variation in abnormal TSH secretion in patients with seasonal affective disorder.
The circadian variations in thyroid-stimulating hormone (TSH) secretion, with particular attention to the nocturnal serum TSH surge and the TSH response to thyrotropin releasing hormone (TRH), were measured in seven patients with seasonal affective disorder (SAD) and in eight normal controls. Both patients with SAD and normal controls were tested in fall/winter, when patients were suffering depressive symptoms, and in spring/summer, when patients were euthymic. The TRH tests were performed in the morning. In all tests, the mean peak TSH response to TRH was significantly lower in the patients with SAD than in the normal controls. No significant differences were observed in either group between spring/summer and fall/winter tests. At both periods, patients with SAD showed normal TSH levels in the morning, but did not experience a nocturnal TSH surge. In this group, morning and night TSH levels were similar. In contrast, normal controls showed significantly higher TSH levels at night than in the morning. Serum-free thyroid hormone levels were in the normal range in all subjects. Morning and night serum cortisol levels and 24-hour urinary cortisol concentrations were similar in all subjects. These data show that the secretion of TSH is impaired in SAD, regardless of the phase of the psychiatric disease. The low TSH response to TRH in the presence of normal serum thyroid hormone levels and the lack of the TSH nocturnal surge suggest that patients with SAD might be affected by mild central hypothyroidism.