Diurnal thyrotropin secretion in short-term profound primary hypothyroidism: Does it ever persist?

Circulating serum thyrotropin (TSH) levels in euthyroid humans show a definite circadian variation, which is maintained in both mild hyperthyroidism and mild hypothyroidism. Yet conflicting data exist with regard to whether this variation persists in at least some patients with severe primary hypothyroidism. We, therefore, studied the diurnal variation in serum TSH in 10 patients (age range 20 to 84 years) with a history of thyroid failure due to prior total thyroidectomy and radioiodine (RAI) ablative treatment performed for thyroid cancer after short-term discontinuation of thyroid hormone (TH) therapy. Serum TSH was measured hourly for a 24-hour period. All data were normalized by converting the TSH values to a percentage (%), designating the 11:00 hour value as 100% (baseline). The average serum TSH levels were markedly elevated in all patients. There was no statistically significant difference between the TSH % values at any time during the 24-hour period when compared with baseline. Further, cosine regression analysis showed absence of rhythmicity in TSH % values over time; notably, no patient showed a variation in TSH % values ≥15% of baseline. In conclusion, diurnal rhythmicity in serum TSH levels was abolished in a uniform cohort of patients with short-term severe primary hypothyroidism. We speculate that the complete lack of peripheral negative feedback input to the hypothalamus or pituitary or both may override the central rhythm-sustaining influences on TSH secretion.