Circadian variations of plasma TSH in patients with severe hypothyroidism.

Five patients with severe hypothyroidism were studied to determine diurnal variations in serum TSH and thyroid hormones. The results were based on blood samples withdrawn at 1-h intervals during a 24-h period. A statistically significant circadian rhythm in TSH was detected with the acrophase at 23(30); no significant diurnal periodicity was observed for serum T3. The TSH pattern was similar to that observed in normal and midly hypothyroid subjects, with low levels during the day and higher levels at night. The findings are consistent with the following concepts: a. the TSH circadian rhythm is retained in patients with severe hypothyroidism; b. the abnormality in thyroid function does not effect the regulation of the TSH periodicity.