Serum thyroxine and triiodothyronine concentrations in neonatal foals and mature horses

Serum thyroxine (T-4) and triiodothyronine (T-3) concentrations were assayed in neonatal foals (1.5 to 4 months) and mature horses (2 to 25 years old) by a modified radioimmunoassay procedure. Blood was collected from 52 clinically healthy foals and horses of various breeds (Thoroughbreds, Quarter Horses, American Saddle Horses, and a single cross-bred horse). Neonatal foals had high serum concentrations of T-4 (mean, 4.02 microgram/dl) and T-3 (192.9 ng/dl) as compared with the values in mature horses (T-4, mean of 1.76 microgram/dl; T-3, mean of 98.69 ng/dl). Stallions had slightly higher T-3; there were no differences between breeds and sexes for serum concentrations of T-4. High serum T-4 and T-3 concentrations in neonatal foals appear to have significant roles in the normal nervous and muscular function and growth in neonatal foals, since hypothyroidism in equine neonates has been reported to show signs similar to those of prematurity in persons, such as neuromuscular incompetence and musculoskeletal dystrophy. Determination of T-4 and T-3 in neonatal foals can be a meaningful screen test for hypothyroidism.