Early development of the thyroid axis in the Brattleboro rat

Plasma concentrations of thyrotrophin (TSH), thyroxine (T4), and triiodothyronine (T3), and pituitary TSH concentrations were determined at weekly intervals during the first 42 days following birth in Brattleboro homozygous (DI), Brattleboro heterozygous (HZ), and Long-Evans (LE) rats. Offspring from matings of Brattleboro rats were divided into DI and HZ animal subgroups on the basis of hypothalamic vasopressin content. In control LE rats, circulating levels of TSH, T4, and T3, and pituitary TSH concentrations increased during the early postnatal period to reach relatively stable levels between 28 and 42 days of age. In DI and HZ rats, the thyroid axis developed in parallel to that of LE rats during initial postnatal weeks. However, by 42 days of age, pituitary TSH concentrations were clearly elevated in Brattleboro rats relative to levels in age-matched LE animals. These data indicate that differences in thyroid axis function between Brattleboro and LE rats occur only after the attainment of a degree of maturity.