| Samuels, Portland, OR, USA, studied the effects of metyrapone on dynamic TSH secretion in 6 healthy subjects. He found that metyrapone leads to a 36% decrease in 24-hour serum cortisol levels, due almost entirely to a 49% decrease in cortisol between 0800h and 1400h. These changes were associated with a 30% increase in daytime TSH levels and thereby abolition of the usual diurnal variation in serum TSH. These results suggest that the usual early morning increase in endogenous cortisol levels may contribute to the normal daytime decrease in TSH levels, while waning evening cortisol levels may permit the normal nocturnal TSH search. TRH tests were also performed with or without metyrapone suggesting that physiological levels of cortisol act via the hypothalamus, rather than directly at the level of the thyrotrope to suppress TSH secretion. |
