Possible mechanism of action of cholesterol enriched diet on the thyroid

Groups of male Sprague Dawley rats aged 60 days were fed either a control diet (CD) or the same diet enriched with 1% cholesterol (CHD) for 30 days. In CHD fed animals an increase of 131I uptake by thyroid, PB131I and TSH level in plasma was found, while the level of thyroxine (T4) was unchanged and that of triiodothyronine (T3) was decreased. In the animals treated with T3 (20 micrograms i.p. daily) a decrease of 131I uptake by thyroid and of T4 and TSH level in plasma was found irrespectively of a cholesterol content in the diet. However, PB131I level was decreased in CHD fed and T3 treated animals compared to those fed CD under the same treatment. Finally, in the animals treated with cimetidine (10 mg i.p. daily) no changes of any measure was found after CD feeding as compared to untreated ones, while CHD in such animals resulted in a decreased of T3 level. The site of cholesterol action appears to be at the hypothalamus-pituitary system since TSH suppressed animals (i.e. T3 treated) did not show any thyroid stimulation when fed with a cholesterol excess. Furthermore, thyroidectomized animals showed increased TSH in plasma after cholesterol feeding. In the present study, chronic administration of cimetidine completely abolished thyroid gland stimulation promoted by cholesterol feeding thus suggesting that central histamine may play a considerable role in this process.