EFFECTS OF A 6-WEEK EXPOSURE TO EXCESS IODIDE ON THYROID-GLANDS OF GROWING AND NONGROWING MALE FISCHER-344 RATS

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 rat s, Ell induced a significant increase in thyroid weight, pituitary wei ght, serum thyroid-stimulating hormone (TS), and thyroxine (T4). The l abeling index (LI) of thyroid follicular cells was slightly increased, although not statistically significant. Histologically, an increase i n follicular cell height, an increase in colloid accumulation, and evi dence of colloid absorption were noted. The effect of bovine TSH (bTSH ) and protirelin tartrate (TRH-t) on LI was significantly augmented by Ell. In nongrowing rats, Ell induced a significant increase in thyroi d 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. Ell did not augment the ef fect of bTSH and TRH-t on the LI of follicular cells. This study sugge sts that growing rats are still susceptible to acute hypothyroidism ev en after 6 wk of continuous exposure to excess iodide, whereas nongrow ing rats are refractory within an equivalent treatment period.