PITUITARY AND THYROIDAL RESPONSES TO SYNTHETIC THYROTROPHIN RELEASING HORMONE IN TWO BREEDS OF CATTLE

The effect of thyrotrophin releasing hormone (TRH) on thyroid and pituitary function has been investigated as a possible aid to the early selection of cattle suited to tropical conditions. Two groups of six calves were used, one a shorthorn breed (SH) adapted to temperate condi-tions and the other an Africander cross (AX) selected for tropical climates. The dose and time responses of serum thyroid-stimulating hormone (TSH), tri-iodothyronine (T3), thyroxine (T4) and prolactin to single, repeated or multiple doses of TRH were measured by radioimmunoassay. Levels of T3 and T4 before injection were lower in the SH than in the AX calves. After a single injection of TRH (0\m=.\4,1, 2\m=.\5or 5 \g=m\g/kglive weight) the percentage increase in T3 in the AX group was significantly lower than that in the SH group. No relationship was observed between the various doses and the magnitude of the response. There was, however, a negative correlation between values of T3 before injection and the maximum T3 response. The T4: T3 molar ratio was also significantly lowered between 2\m=.\5and4\m=.\5h after injection of TRH. Changes in prolactin but not TSH concentrations were dependent upon the breed. Afte