Deficient Nocturnal Surge of Thyrotropin in Central Hypothyroidism

In normal individuals, serum TSH concentrations have a circadian pattern characterized by a nocturnal surge which begins in the late afternoon and reaches its peak after midnight. We assessed the nocturnal surge of TSH in 16 patients with pituitary and/or hypothalamic diseases, 6 of whom were judged to be hypothyroid. To assess the magnitude of the nocturnal surge in individual patients, TSH was measured in 5 serum samples obtained during the normal time of the TSH nadir in the late afternoon and in 5 samples obtained during the normal time of the peak of serum TSH after midnight. A significant nocturnal surge of TSH was defined as a significantly greater mean nighttime TSH level than the mean daytime TSH concentration. The nocturnal TSH surge was absent in the 6 patients with central hypothyroidism, while it was present in the 10 euthyroid patients with central lesions. In 6 hypothyroid patients who did not have pituitary or hypothalamic lesions, the nocturnal TSH surge was intact, indicating that hypothyroidism per se does not account for the deficient nocturnal TSH surge in central hypothyroidism. We conclude that central hypothyroidism is characterized by a deficient nocturnal surge of TSH, and accordingly, we suggest that evaluation of the circadian pattern of TSH may be a useful adjunct in making the diagnosis of hypothyroidism in patients with diseases involving the pituitary or hypothalamus.