AJP - Endocrinology and Metabolism, Vol 239, Issue 3 223-E226, Copyright © 1980 by American Physiological Society
Maturation of hypothalamic-pituitary-thyroid response in the rat to acute cold
S. Frankel and G. Lange
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.
