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

Publisher Website
Google Scholar
Abstract
Groups of rats 3, 4, 5, 7 and 10 wk of age were tested for their ability to respond to acute cold (4.degree. 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 and 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. In rats, hypothalamic-pituitary-thyroid reactions to acute cold attain the adult pattern between 5 and 7 wk after birth.