Pituitary reserve after repeated administrations of releasing hormones in young and in elderly men: Reproducibility on different days
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Combined testing of the pituitary gland by administration of GHRH, CRH, GnRH and TRH has been proposed for clinical studies, although some reports indicate that individual endocrine responses can be different when releasing hormones are used alone or in combination. Aims of this study were to evaluate: 1) the reproducibility of the combined test on different days; 2) the endocrine responses to the combined test applied twice at 3h and at 5h interval; 3) differences of endocrine responses in young and in elderly men. Six healthy young men (aged 25 to 35 yr) and 6 healthy elderly men (aged 65 to 75 yr) were evaluated: elderly men had lower testosterone, free T3, and somatomedin-c levels than young men, while 17ß-estradiol and inhibin were not significantly different, all values being within normal laboratory limits. The 12 men were tested on day 1 with iv GHRH (50 μg) CRH (50 μg), GnRH (100 μg) and TRH (200 μ g) at 08:00h and again at 11:00h; on day 8, the same men were tested at 08:00h and again at 13:00h. At 08:00h, the endocrine responses were similar on day 1 and on day 8. The second GH (young and elderly men) and PRL (only young men) response was blunted on day 1, when the interval between two consecutive stimuli was 3h, but not on day 8, when the interval was 5h. Elderly men differed from young men only for GH and for PRL release on day 1 at 08:00h. Because of a similar behaviour, young and elderly men were then considered together; the second LH and TSH response was blunted on day 1, and the second LH and FSH response was blunted on day 8. The second Cortisol response was usually higher than the first response, the difference being statistically significant on day 1, not on day 8. These data indicate that: 1) the combined administration of the four releasing hormones is reproducible on different days. 2) when two consecutive stimuli are applied, the time required by each endocrine response to return to normal is different; 3) elderly men had a reduced GH and PRL release in comparison to young men; 4) the fact that elderly men had a FSH, LH, and TSH release similar to young men in spite of lower free T3 and testosterone levels speaks in favour of a reduced, or nonproportional, hypothalamic/pituitary endocrine function in healthy elderly men.
Key-wordsAging pituitary gland somatotropin LH FSH thyrotropin prolactin Cortisol inhibin physiology somatomedins 17ß-estradiol testosterone
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