Seasonal influence on thyroidal response to thyrotrophin-releasing hormone in cattle and buffaloes.

The effect of thyrotrophin-releasing hormone (TRH) on the circulating plasma levels of tri-iodothyronine (T3) and thyroxine (T4) was determined in the same group of animals (four cattle and four Murrah buffaloes) during hot dry (HD), hot humid (HH) and cold environmental conditions. Plasma T3 and T4 concentrations were measured during 2 h before and up to 12 h after the administration of TRH (200 micrograms i.v.). In the preinjection period in both cattle and buffaloes T3 levels were significantly lower in HH conditions. No significant difference in basal (preinjection) T3 levels was observed during HD and cold seasons in cattle. The highest T3 levels were obtained in buffaloes during HD season with intermediate values during the cold months. Plasma T4 levels in these animals were reversed during HD and HH months. In both cattle and buffaloes there was a biphasic response of T3 and T4 to TRH treatment and this varied with time and in size. The season significantly affected the T3 response to TRH in cattle and buffaloes but the T4 response differed in the two species. The ratio of T4/T3 was higher during HH condition compared with other seasons in both cattle and buffaloes. The climate significantly affected the thyroidal response to TRH.