Circadian changes in pulsatile TSH release in primary hypothyroidism.

OBJECTIVE

We evaluated pulsatile and circadian TSH secretion in primary hypothyroidism.

DESIGN

In a prospective study, blood was sampled every 10 minutes during 24 hours for assay of TSH (IRMA). Thyroid hormones and TSH responsiveness to TRH were then measured.

SUBJECTS

Nine patients with overt primary hypothyroidism, seven patients with subclinical hypothyroidism and 16 healthy controls.

MEASUREMENTS

Computer-assisted analysis by the Desade and Cluster programs.

RESULTS

Both computer-assisted programs revealed an increased TSH pulse amplitude in both overt and subclinical hypothyroidism versus controls (Desade: 36.9 +/- 31.4 (mean +/- SD) (P < 0.001) and 2.8 +/- 1.9 (P < 0.001) vs 0.4 +/- 0.2 mU/l; Cluster: 25.6 +/- 25.1 (P < 0.001) and 2.4 +/- 1.4 (P < 0.001) vs 0.4 +/- 0.2 mU/l). TSH pulse frequency remained unchanged with approximately 10 pulses/24 hours. A highly significant correlation was found between the mean 24-hour TSH concentration and the TSH pulse amplitude in all controls and patients but not to TSH pulse frequency. The nocturnal TSH surge was absent in six out of nine patients with overt primary hypothyroidism. The deficient nocturnal rise of TSH in primary hypothyroidism vs controls (22 +/- 51 vs 82 +/- 41%, P < 0.001), was associated with a loss of the usual nocturnal increase in TSH pulse amplitude and frequency.

CONCLUSIONS

Mean 24-hour TSH pulse amplitude is increased in primary hypothyroidism, but TSH pulse frequency remains unchanged. The decrease of the nocturnal TSH increase in primary hypothyroidism is associated with a loss of the usual nocturnal increase in TSH pulse amplitude and frequency.