AGE-RELATED MODIFICATIONS IN THE REGULATION OF THE HYPOTHALAMIC-PITUITARY-THYROID AXIS

We studied the hypothalamic-pituitary-thyroid function in two groups o f healthy elderly subjects: group A (n = 23, age range 65-80 years), a nd group B (n = 11, age range 81-92 years), and in 32 controls, aged 2 0-60. A TRH test for TSH and prolactin was performed in all subjects, while the TSH circadian modulation was evaluated in elderly subjects o nly. Group B showed significantly lower fT(3) and TSH, and higher fT(4 ) levels with respect to controls (fT(3): 4.4 +/- 0.2 vs. 5.2 +/- 0.2 pmol/l, p < 0.05; fT(4): 13.1 +/- 0.9 vs. 11.4 +/- 0.4 pmol/l, p < 0.0 5; TSH: 1.07 +/- 0.21 vs. 1.46 +/- 0.13 mIU/l, p < 0.05). Morning TSH showed an inverse correlation with age (r = -0.42; p < 0.02) among the 34 elderly subjects, but not among controls. Evidence for TSH circadi an modulation was found only in group A (nighttime TSH: 1.60 +/- 0.17, vs. daytime: 1.25 +/- 0.13 mIU/l, p < 0.001). The TRH-stimulated TSH peak was reduced among all elderly subjects with respect to controls ( A: 6.26 +/- 0.64 mIU/l, p = 0.01; B: 5.02 +/- 0.58 mIU/l, p < 0.01). T he maximal PRL response was also blunted (A: 25.7 +/- 2.6 mu g/l, B: 2 7.7 +/- 5.2 mu g/l, p < 0.0005). In conclusion, a resetting of the pit uitary threshold of the TSH feedback suppression, along with complex a lterations in peripheral thyroid hormone levels, may progressively dev elop in older people, becoming apparent only with extreme senescence. Moreover, the TSH nocturnal surge may be lost with increasing age, thu s providing evidence also for hypothalamic dysfunction.