Synthetic thyrotropin releasing factor as a test of pituitary thyrotropin reserve

The iv administration of 0.5 mg of synthetic thyroptropin releasing factor (TRF) to control subjects increased plasma TSH from baseline levels of 2.1 ± 0.01 µU/ml to peak values of from 8.5-27 µU/ml. Plasma TSH levels in subjects with primary myxedema were elevated and rose further following TRF. In seven of 16 euthyroid subjects with pituitary tumors, peak values of plasma TSH following TRF ranged from 2.5-6.5 µU/ml indicating limited reserve for TSH secretion. Subjects with hypothyroidism secondary to Sheehan’s syndrome or pituitary tumors did not alter their plasma TSH levels following TRF indicating inadequate pituitary function. Plasma TSH rose normally following TRF in five subjects with TSH deficiency in the absence of a known organic pituitary lesion. This latter group of subjects may have TRF deficiency as a cause of their decreased TSH secretion.