Effects of dietary energy supply on serum thyroxine, tri-iodothyronine and insulin concentrations in young horses.

The effects of meal ingestion on the circulating concentrations of the growth-regulating hormones thyroxine (T4), tri-iodothyronine (T3) and insulin were examined in weanling Thoroughbreds fed 70% (diet A), 100% (diet B) or 130% (diet C) of their energy and protein requirements. Peak insulin concentrations occurred 1, 2 and 3 h after the ingestion of diets C, B and A respectively. Increases in plasma glucose concentrations preceded the increases in serum insulin concentrations. Serum T4 concentrations increased after the ingestion of diets A and B and decreased after diet C. In contrast, serum T3 concentrations were unaffected by ingestion of diet A but increased after the ingestion of diets B and C. The increase was much greater and more rapid in the horses fed diet C. However, the decrease in T4 concentration was five times greater than the increase in T3 concentration. Accelerated insulin secretion after the ingestion of a meal high in energy (carbohydrate) content was therefore associated with decreased T4 secretion and accelerated T4 conversion to T3. However, 6 months later serum T4 and T3 concentrations were unaffected by meal ingestion.