The hypothalamic-pituitary-thyroid axis and melatonin in humans: possible interactions in the control of body temperature.

OBJECTIVE: Melatonin plays a role in the regulation of biological rhythms, body temperature presents circadian variations with lower levels during nighttime, when melatonin levels are very high, and thyroid hormones influence shiver independent thermogenesis. We have investigated on possible interactions between the hypothalamic-pituitary-thyroid axis and melatonin in the control of body temperature in humans.

METHODS: Peripheral blood samples for thyrotropin-releasing hormone (TRH), thyroid-stimulating hormone (TSH), free-thyroxine (FT4), melatonin levels determination and body temperature measurements were obtained every four hours for 24-hours starting at 0600 h in a controlled temperature and light-dark environment from ten healthy males, aged 38-65 (mean age +/-s.e. 57.4+/-3.03, mean body mass index +/-s.e. 25.5+/-0.75). We calculated fractional variation and correlation on single time point hormone serum levels and tested whether the time-qualified data series showed consistent pattern of circadian variation.

RESULTS: A statistically significant difference was evidenced for the fractional variation of daytime TSH serum levels (0600 h-1000 h vs. 1000 h-1400 h, p=0.01, 1000 h-1400 h vs. 1400 h-1800 h, p=0.0001, 1400 h-1800 h vs. 1800 h-2200 h, p=0.001) and for the fractional variation of FT4 serum levels at 1800 h-2200 h vs. 2200 h-0200 h (p=0.02). FT4 serum levels correlated positively with TRH serum levels at 1000 h (r=0.67, P=0.03) and at 1400 h (r=0.63, p=0.04), negatively with TSH serum levels at 2200 h (r=-0.67, p=0.03), negatively with melatonin serum levels at 2200 h (r=-0.64, p=0.04) and at 0200 h (r=-0.73, p=0.01). TRH serum levels correlated positively with TSH serum levels at 0200 h (r=0.65, p=0.04) and at 0600 h (r=0.64, p=0.04). Body temperature correlated positively with FT4 serum levels at 1000 h (r=0.63, p=0.04) and negatively with melatonin serum levels at 0200 h (r=-0.64, p=0.04). A clear circadian rhythm was validated for body temperature (with acrophase in the morning) and melatonin, TRH and TSH secretion (with acrophase at night), while FT4 serum level changes presented ultradian periodicity (with acrophase in the morning).

CONCLUSION: Changes of TSH serum levels are smaller and those of FT4 are greater at night, when melatonin levels are higher, so that the response of anterior pituitary to hypothalamic TRH and of thyroid to hypophyseal TSH may be influenced by the pineal hormone that may modulate the hypothalamic-pituitary-thyroid axis function and influence the circadian rhythm of body temperature.