Thyroid and Parathyroid Disorders in Horses

Unusually low or high levels of thyroid hormones are seen rarely in horses. A low resting level of T3 and T4 hormones does not necessarily mean that these hormones are not being produced in adequate amounts because the low level may be related not to thyroid function but to other illness or administration of drugs such as phenylbutazone.

Three hormones regulate calcium levels within the horse’s body. Parathyroid hormone increases retention of calcium through kidney action, and also stimulates release of calcium from the skeleton. Calcitrol promotes calcium absorption, while calcitonin stimulates calcium loss by the kidneys and supports conservation of calcium in the skeleton.

Hyperparathyroidism results from excessive parathyroid hormone and causes weight loss, shifting lameness, depression, and loss of appetite. It may be caused by a tumor, kidney failure, or ingestion of certain plants such as yellow oatgrass or waxyleaf nightshade.

Secondary hyperparathyroidism, or big-head disease, can result from a diet that is too low in calcium and too high in phosphorus. The horse’s body releases calcium from the skeleton to try to balance these minerals, and lameness, a stiff gait, and enlargement of the bones of the horse’s head may be seen.

Hypoparathyroidism is rare but is occasionally seen in foals and adult horses. Gait abnormalities, synchronous diaphragmatic flutter, and muscle twitching or tremors may be seen in horses with this condition. The cause may be difficult to identify, but treatment with increased dietary calcium and vitamin D is helpful in some cases.

If your horse shows disease signs or physical changes, consult a veterinarian and possibly an equine nutritionist to identify the problem and get management guidelines.