Hypothyroidism has been well documented though still poorly understood disease in newborn foals. This article discusses the clinical signs, diagnosis, treatment, prevention, and prognosis of this disease and contains recent scientific reports on this problem.

Hypothyroidism in adult equines is considerably different, ...more.

Clinical signs are dependent on the severity, duration, and timing of the hypothyroidism:

• Mild or early hypothyroidism results in goiter, enlargement of the thyroid gland, and may be the only symptom.
• Moderate hypothyriodism late in pregnancy usually results is a hypometabolic state with incoordination, poor sucking reflexes, hypothermia, and goiter.
• Severe hypothyroidism during the early and middle stages of pregnancy results is developmental muscoskeletal deformities and lesions. This form is most often seen in western Canada, and presents as skeletal deformities, flexural deformities of the legs, ruptured common digital extensor, incomplete ossification of the bones resulting in angular deformities.

More Information on The summary of a 1994 AAEP presentaion by Peter B. Fretz, VMD on this syndrome is: Introduction A syndrome of congenital thyroid gland hyperplasia with a consistent pattern of musculoskeletal lesions, including mandibular prognathia, flexural deformities of the front legs, ruptured digital extensor tendons, and incompletely ossified carpal and tarsal bones, have been reported in western Canadian foals.1-4 With the exception of what appears to be the same disease syndrome described in the Minnesota, Wisconsin area of the U. S. in 1975,5 this problem had not been reported outside of western Canada. Materials and Methods Investigations into this syndrome were renewed in 1990, and findings from the following are summarized: (a) a retrospective study of the records (1980-1989) of eight diagnostic veterinary laboratories in western Canada, (b) examination of 17 cases presented to the Western College of Veterinary Medicine-Large Animal Clinic and/or diagnostic laboratory, and (c) investigations of farms experiencing a high incidence of the disease, conducted between 1990 and 1992. Results Affected foals were found to be hypothyroid through the use of thyroid-stimulating response tests, while their dams were found to have normal thyroid function at the time of parturition.7 All affected foals were delivered after a prolonged gestation with a mean of 360 days and a range of 340-400 days. In spite of the long gestation, foals have several signs or lesions of immaturity, including (a) a short, silky haircoat, (b) soft, pliable ears, (c) lax tendons and joints, (d) generalized musculoskeletal weakness, (e) small size, and (f) incomplete skeletal development. These foals are referred to as dysmature. A diagnosis of congenital hypothyroidism and dysmaturity is made when there are appropriate history and clinical findings or lesions, including a low skeletal ossification index.8. There is no sex or breed predisposition associated with the disease, and multiple cases usually occur on an affected farm during the same foaling season. The cause of the syndrome remains unknown. Most foals die or.are euthanized during the first week of life. Foals that survive and have been followed until several weeks to several months of age acquire a variety of developmental orthopedic diseases, including angular limb deformities, tarsal collapse, physitis, and lesions associated with generalized osteochondrosis.

Because the fetus and foal are more sensitive to the effects and causes of hypothyroidism the mare may not show any signs of problems.