Effects of a six-week exposure to excess iodide on thyroid glands of growing and nongrowing male Fischer-344 rats.

Abstract

A 6-wk exposure to excess iodide intake (EII) via drinking water (260 mg potassium iodide/L) demonstrated different effects on growing (4-wk old) and nongrowing (45-wk old) male Fischer-344 rats. In growing rats, EII induced a significant increase in thyroid weight, pituitary weight, serum thyroid-stimulating hormone (TSH), and thyroxine (T4). The labeling index (LI) of thyroid follicular cells was slightly increased, although not statistically significant. Histologically, an increase in follicular cell height, an increase in colloid accumulation, and evidence of colloid absorption were noted. The effect of bovine TSH (bTSH) and protirelin tartrate (TRH-t) on LI was significantly augmented by EII. In nongrowing rats, EII induced a significant increase in thyroid weight and serum T4 but no increase in pituitary weight, serum TSH, and the LI of follicular cells. Histologically, an increase in colloid accumulation was found in small follicles. EII did not augment the effect of bTSH and TRH-t on the LI of follicular cells. This study suggests that growing rats are still susceptible to acute hypothyroidism even after 6 wk of continuous exposure to excess iodide, whereas nongrowing rats are refractory within an equivalent treatment period.