Nocturnal melatonin secretion in thyroid disease and in obesity

The purpose was to investigate whether the function of the pinealocytes is changed in patients with disturbed metabolic rate or in subjects with marked obesity.

Venous blood was sampled every second hour between 1800 and 0800 h, and urine collected between 2200 and 0700 h.

Eight patients with thyrotoxicosis, six with primary hypothyroidism, eight with maturity-onset obesity, and 12 healthy controls were included in the study.

Peak serum melatonin values during the night, total nocturnal melatonin secretion (estimated by melatonin incremental areas), and urinary excretion of melatonin were determined in each participant.

Patients with hypothyroidism were found to have higher peak serum melatonin values, total nocturnal melatonin secretion, and urinary excretion of melatonin than normal individuals (Peak melatonin values: 0.55 +/- 0.13 vs 0.27 +/- 0.04 nmol/l, respectively; P less than 0.05. Melatonin incremental areas: 3.38 +/- 0.80 vs 1.45 +/- 0.26 nmol/l h; P less than 0.05. Melatonin excretion: 0.140 +/- 0.023 vs 0.081 +/- 0.013 nmol/9 h; P less than 0.05). Neither of these values differed significantly from the normal in patients with thyrotoxicosis or obesity. Although thyrotoxic patients thus released a normal amount of melatonin during the night, their melatonin secretion peak was phase advanced (melatonin secretion peak appearing at 1.1 +/- 0.5 h in thyrotoxic, and at 3.4 +/- 0.5 h in normal participants; P less than 0.01). No such phase shifts were seen in patients with obesity or hypothyroidism.

These findings imply that both hypothyroid and thyrotoxic patients have disturbed pineal function, which is not the case in patients with obesity. The mechanism underlying these observations remains to be elucidated.