Ethanol inhibition of pituitary-thyroid axis: an effect secondary to nutritional deficiency.

Ethanol as either 20% or 36% of total calories in a Lieber diet was administered to male rats. At these concentrations, ethanol consumption relative to body weight did not differ. Pair-fed controls were restricted to the amount of calories consumed by rats given ethanol. Under these conditions, a direct effect of ethanol on the hypothalamic-hypophyseal-thyroid axis could not be demonstrated. There were no differences between pair-fed control and ethanol treated rats in serum or pituitary TSH, TSH response to TRH, or T4 and T3 levels. On the other hand, in rats given ethanol as 36% of total calories ("36%" ethanol-treated), and in their pair-fed controls, a marked decrease in serum T4 levels occurred (25% and 30%), relative to the corresponding "20%" groups. The decreased T4 in the "36%" groups was associated with a pronounced fall in caloric intake, decreased serum TSH, and declines in adenohypophyseal and body weights -- all of which were of similar magnitude in experimental and control rats. Thus, inanition was probably the primary cause of reduced thyroid function in the "36%" groups. An interesting aspect of this change was the finding of no difference in serum T3 levels between pair-fed control and ethanol treated rats in the 36% and 20% groups despite the reduced T4 and caloric intake in 36% animals; the lack of decrease in T3 concentration in 36% animals may reflect augmented peripheral conversion of T4 to T3 or reduced T3 clearance.