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

Abstract

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.

Measurement of Free Thyroxine Concentration in Horses by Equilibrium Dialysis

“…60 In several other studies of adult or weanling horses, energy or protein-restricted diet also failed to alter daily serum T 3 or T 4 concentration. [61][62][63] Some of the ill horses in this study had unexpectedly high serum concentration of fT 4 D. This phenomenon also has been reported in cats with nonthyroidal illness syndrome 38,64 and in humans. 48,65 Increased measurement of fT 4 concentration by dialysis in ill patients may truly represent an in vivo phenomenon (eg, by alteration of protein binding or decreased conversion of T 4 to T 3 ), or may represent an artifact of the measurement technique.…”

Total and free iodothyronine levels of growing Thoroughbred foals: Effects of weaning and gender

“…In fact, the weaning results in the deprivation of maternal care, often social isolation, and a sudden transition in the source of nutrient (Hoffman et al, 1995;Malinowski et al, 1996;Moons et al, 2005). No differences in plasma T 3 and T 4 concentrations in weanling horses fed restricted diets were reported (Glade et al, 1984;Glade and Reimers, 1985), whereas in other species, reduced intakes decreased thyroid hormone concentrations (Ellenberger et al, 1989;Buonomo and Baile, 1991;Janan et al, 1995). In addition, the development of diurnal rhythms of thyroid hormones was observed in foals from birth till the end of the first year of their life (Komosa et al, 1990), but the environmental conditions and physiological state of an organism may modify the course and amplitude of rhythms.…”

Effect of fast vs. moderate growth rate related to nutrient intake on developmental orthopaedic disease in the horse

“…More recently, Savage et al [34] demonstrated that high energy (but not protein) allowances, dramatically increases OC incidence. These lesions could be mediated by overfeeding induced insulin or/and thyroid hormone postprandial secretion pattern changes [11,30]. Investigations in trace minerals have been spurred by reports that copper and zinc supplementation could decrease the incidence of DOD [19], and the incidence is increased by severe copper deficiency [2].…”