Thyrotropin Levels during Hydrocortisone Infusions That Mimic Fasting-Induced Cortisol Elevations: A Clinical Research Center Study

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Abstract
Both short term fasting and administration of high doses of glu- cocorticoids lead to marked suppression of serum TSH levels in healthy subjects. However, it is not known whether the more mild serum cortisol elevations seen during fasting can account for fasting- induced TSH suppression. To study this question, eight healthy sub- jects each underwent three 2-day studies: 1) baseline (ad libitum diet), 2) fasting (56 h of total caloric deprivation), 3) hydrocortisone (HC) infusions at a dose and pulsatile pattern that reproduced cortisol levels measured during each subject's fasting study. Subjects re- quired 34 - 46 mg HC/24 h to achieve these cortisol levels. During each study, blood samples were drawn every 15 min during the final 24 h for serum cortisol and TSH levels. A TRH stimulation test was per- formed at the end of each study. By design, fasting and HC infusions induced similar mild increases in 24-h serum cortisol levels (32% over baseline), with the most significant increases seen between 1400 - 0200 h. Fasting decreased 24-h mean and pulsatile TSH levels 65% from baseline, whereas HC infusions decreased mean and pulsatile TSH levels 51% from baseline. Daytime (0800 - 0200 h) TSH levels were identical in the two studies, whereas nocturnal (0200 - 0800 h) TSH levels during HC infusions fell midway between baseline and fasting studies. Serum total T3 and TSH responses to TRH were decreased to a similar degree by fasting or HC infusions. These results suggest that mild elevations in endogenous cortisol levels may me- diate at least in part fasting-induced changes in TSH secretion and thyroid hormone levels. In addition, these data show that near-phys- iological doses of HC and resulting changes in serum cortisol levels within the normal range can cause significant decreases in serum TSH levels. (J Clin Endocrinol Metab 82: 3700 -3704, 1997)

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