Maturation of hypothalamic-pituitary-thyroid response in the rat to acute cold

Groups of rats 3, 4, 5, 7, and 10 wk of age were tested for their ability to respond to acute cold (4 degrees C for 30 min). Thyroid reactions were estimated from intrathyroidal colloid droplet (CD) counts, serum thyrotropin (TSH) and thyroxine (T4) levels. Normally CD counts increased to a maximum in the 5-wk-old rats were lower in the 7-wk-old, and still lower in the 10-wk-old animals. The significant finding was that exposure to cold caused a decrease in CD counts in 3-, 4-, 5-wk-old rats but an increase in mature animals. This CD decrease was apparent within 10 min in the 4- and 5-wk-old rats, Injections of TSH and thyrotropin-releasing hormone (TRH) raised CD counts in all age groups but had little effect on response to cold. TSH serum levels were higher in the 5-wk-old than in the 3- and 4-wk-old animals; cold caused a greater TSH reduction in the 5-wk-old rats than in younger groups. Serum T4 levels were approximately the same in 3-, 4-, and 5-wk-old rats; when they were exposed to cold, the level rose despite the fact that CD counts dropped. These data indicate that in rats hypothalamic-pituitary-thyroid reactions to acute cold attain the adult pattern between 5 and 7 wk after birth.