A foal’s thyroid sets it apart from many other youngsters–the sky-high levels of hormone it secretes allow the foal to be darting around the stall within hours of birth, rather than it remaining immobile and nursing with its eyes shut for weeks like other mammalian neonates. Researchers are striving to better understand the neonatal thyroid’s function and dysfunction, said Babetta A. Breuhaus, DVM, PhD, an associate professor of equine medicine at North Carolina State University, in her June 2 presentation at the American College of Veterinary Internal Medicine Forum in Louisville, Ky.

Thyroid hormones are important for growth, maturation of organ systems, and regulation of metabolism, said Breuhaus. This maturation of organ systems results in increased mental activity and neural development in the fetus and neonate, lung maturation, increased gastrointestinal function, and increased cardiovascular function. Thyroid hormones can be detected in the fetus by mid-gestation, and the levels increase gradually as the delivery date approaches.

Equine neonates have much higher serum thyroid hormone concentrations at birth than other species that have been studied. “This rise in hormone has been shown in a number of species, most before birth” said Breuhaus. “In the foal, it’s a little delayed, but the higher levels seem to be exaggerated. The high levels at birth could explain why they (foals) are born so developmentally mature compared to a puppy or kitten.

“Thyroid hormones in the foal are in the neighborhood of 10 times the normal amount in an adult horse,” Breuhaus continued. This could be due to placental stimulation, a link to the mare’s milk, or a different receptor activity that down-regulates the hormones as the horse gets older