Methimazole-induced hypothyroidism in rats: Effects on body weight and histological characteristics of thyroid gland

Summary: The aim of this study was to examine the effect of methimazole treatment on the body weight and thyroid gland structure in rats. Methimazole given as 0.02% solution in drinking water for three weeks induced significant decline in T4 and T3 levels, as determined by radioimmunoassay. The body weight gain was lowered compared to control animals, while thyroid weight was increased. Histological examination of the thyroid gland revealed a pronounced growth activation of the follicular epithelial component with frequent mitoses, accompanied with improved vascularisation. We assumed that the lower body weight gain despite decreased basal metabolic rate and similar food ingestion can be a result of brown adipose tissue activity.