THE NOCTURNAL THYROID-STIMULATING HORMONE SURGE IS ABSENT IN OVERT, PRESENT IN MILD PRIMARY AND EQUIVOCAL IN CENTRAL HYPOTHYROIDISM

The nocturnal TSH surge was studied in controls, in 34 patients with hypothalamic/pituitary disease and in 21 patients with primary hypothyroidism. It was absent in 5/12 hypothyroid patients and in 5/22 euthyroid patients with hypothalamic/pituitary disease (42% vs 23%, NS). Central hypothyroidism relative to euthyroidism was associated with a lower absolute (0.3 +/- 0.4 vs 0.9 +/- 1.0 mU/l, p < 0.05) and relative (24 +/- 31 vs 63 +/- 51%, p < 0.05) nocturnal rise in TSH. In primary hypothyroidism, the nocturnal TSH surge was absent in eight of ten patients with overt, in one of five patients with mild and in none of six patients with subclinical hypothyroidism. The relative nocturnal rise in TSH was normal in mild (54 +/- 33%) and subclinical (92 +/- 69%), but decreased in overt hypothyroidism (2 +/- 10%). Plasma T4 was positively and 09.00 plasma TSH negatively related to the relative nocturnal TSH surge in primary hypothyroidism, but not in central lesions. In both conditions, however, a positive relationship was observed between the relative nocturnal TSH surge and the relative increase of TSH to TRH. In conclusion: (a) The nocturnal TSH surge is usually absent in overt hypothyroidism but present in mild primary hypothyroidism and equivocal in central hypothyroidism. This limits its usefulness as an adjunct in the diagnosis of central hypothyroidism. (b) The magnitude of the nocturnal TSH surge in patients with hypothalamic/pituitary disease or primary hypothyroidism is directly related to the TSH response to TRH, and thus appears to be determined by the directly releasable TSH pool of the pituitary.