Effect of thyroid status modulation on pituitary and peripheral hormone concentrations in healthy older subjects.

Introduction: Depending on age, sex, and familial longevity, alterations in thyroid status occur frequently, and often co-occur with differences in other hormonal axes. However, studies that explore the effects of thyroid status modulation on other hormonal axes remain scarce. We aim to determine the effects of thyroid status modulation on prolactin, IGF-1, cortisol, LH, testosterone, and SHBG levels. We also explored whether effects differed depending on type of challenge, sex, and familial longevity.

Methods: Data was gathered from two single-arm challenge studies comprising an intramuscular injection of 0.1 mg recombinant human (rh)TSH (N=29) or 100 µg T3 orally (N=27) in healthy older individuals. Changes in hormone concentration profiles relative to baseline were determined for 4 and 5 days respectively.

Results: IGF-1 increased with a maximum of 6.3% (SEM=1.6%, P=0.002) in the rhTSH challenge and 8.8% (SEM=1.6%, P<0.001) in the T3 challenge, while LH (19.3% (SEM=6.6%, P=0.048)), testosterone (13.8% (SEM=4.7%, P=0.048)), and SHBG (11.8% (SEM=3.5%, P=0.02)) increased significantly in the T3 challenge only. Moreover, prolactin significantly decreased in both rhTSH and T3 challenges (-8.8% (SEM=3.4%, P=0.048) and -12.0% (3.3%, P=0.004) respectively) as did cortisol (-14.8% (SEM=3.6%, P<0.001) and -15.6% (SEM=3.5%, P<0.001)). There was no significant interaction with type of challenge, sex, or familial longevity, except for prolactin in the rhTSH challenge (P=0.004) which decreased significantly in men only.

Conclusions: Upon modulation of thyroid status, changes were observed in IGF-1, prolactin, and cortisol. In the T3 challenge, LH, testosterone, and SHBG increased in men. Observed changes are hypothesized to be driven by (f)T3.