The effects of partial thyroidectomy on the development of the equine fetus.

A syndrome of congenital hypothyroidism and dysmaturity has been an important cause of reproductive loss and foal mortality in western Canada. The cause and pathogenesis of this syndrome is under investigation. One issue to be addressed is whether all the anomalies present in affected foals are produced concurrently by the same agent, or if affected foals are primarily hypothyroid in utero which induces the associated lesions. This study was designed to document the effects of fetal thyroidectomy, at about 215 days of gestation, on the growth and development of the equine fetus and to compare the anomalies present to those reported in the spontaneously occurring syndrome of foals in western Canada. Two sham-operated controls and 4 partially thyroidectomised foals were carried to term following surgery. Sham-operated control foals were normally developed. Partially thyroidectomised foals were hypothyroid; had hyperplastic thyroid gland remnants; abnormal behaviour and locomotor skills; and numerous, marked deficiencies in their skeletal development. The anomalies present in partially thyroidectomised foals were comparable to those reported in congenitally hypothyroid neonates of other species, including human infants, and were similar to those described in congenitally hypothyroid and dysmature foals reported in western Canada.