Serum Thyrotropin (TSH) in Pituitary and/or Hypothalamic Hypothyroidism: Normal or Elevated Basal Levels and Paradoxical Responses to Thyrotropin-Releasing Hormone1
Using a highly sensitive and specific radioimmunoassay for thyroid-stimulating hormone (TSH), basal serum TSH levels and the response to synthetic thyrotropin-releasing hormone (TRH) were evaluated in 21 patients with unequivocal hypothyroidism resulting from pituitary or hypothalamic lesions. Baseline TSH concentrations were undetectable in only 2, being normal (n = 9) or slightly elevated (n = 10) in the rest. Three of those with elevated levels had evidence of associated primary thyroid disease. Excluding these, the mean serum TSH concentration in the remaining 18 patients (3.4 ± 0.8 (sem) μU/ml) was significantly higher (p < 0.0005) than the mean of 75 euthyroid control subjects (1.0 ± 0.1 μU/ml). Patients with hypothalamic lesions generally had higher basal TSH levels than those with pituitary lesions. Treatment with thyroxine 0.1–0.3 mg/day led to suppression of the measured TSH values. Dilutions of sera containing normal and elevated TSH levels, showed parallelism with the standard curve (MRC Standard A). The administration of 200 μg iv synthetic TRH to this group of patients evoked a serum TSH response characterised by a peak of normal magnitude in 12 of the 20 patients tested. Six patients showed an exaggerated response (including 3 with concomitant thyroid disease). An impaired response was observed in only 2 instances. It is concluded from this study that: 1. (a) Clearly detectable and frequently mildly elevated basal immunoreactive serum TSH levels commonly occur in patients with pituitary and hypothalamic hypothyroidism. 2. (b) TSH reserve is usually normal in hypothalamic and pituitary hypothyroidism and TRH responsiveness cannot adequately differentiate between the 2 conditions.