Hypothyroidism refers to inadequate production of thyroid hormones, primarily thyroxine and (T4) and triiodothyronine (T3). These hormones are produced by the thyroid gland and play a role in regulating your horse’s metabolism.

Hypothyroidism most commonly occurs in neonatal foals and is rarely observed in adult horses. [1]

In foals, the condition results in musculoskeletal deformities and, in some cases, thyroid gland enlargement. [2] When hypothyroidism occurs in adult horses, typical signs include dull coat, lethargy, and poor performance. [3]

Neonatal hypothyroidism is believed to occur when mares fail to consume sufficient iodine during gestation. [2] Excess nitrate intake and the consumption of plants that interfere with thyroid function are also proposed causes of the condition. [2]

Diagnosing hypothyroidism requires testing levels of thyroid hormones including T4 and T3 and carefully considering clinical signs. [3] A more accurate diagnosis can be made in combination with the thyrotropin-releasing hormone (TRH) stimulation test. [3]

Treatment of hypothyroidism in adult horses requires identifying and addressing the underlying cause of the condition. There is no treatment for neonatal hypothyroidism. Ensuring optimal nutritional status of pregnant mares during gestation is critical for improving the chance of healthy foals at birth.

What is Hypothyroidism in Horses?

Hypothyroidism is a hormonal imbalance caused by the insufficient production of biologically active thyroid hormone (TH) from the thyroid gland.

When hypothyroidism occurs in horses, the condition most commonly presents in neonatal foals. [4][5] Naturally occurring cases of hypothyroidism in horses beyond the neonatal stage are extremely rare.

In the limited case reports of adult horses diagnosed as hypothyroid (in which the condition was not induced), clinical signs primarily included lethargy, exercise intolerance, and poor hair coat. [3]

Health issues including laminitis and weight gain are sometimes misdiagnosed as hypothyroidism. Measuring low thyroid hormone levels may also be due to other illness and natural variations in thyroid hormone production.

Types of Hypothyroidism

Hypothyroidism in horses can be classified as a primary, secondary, or tertiary disease. Primary hypothyroidism describes diseases involving the thyroid gland itself. [1]

Experimentally, primary hypothyroidism (diagnosed according to laboratory tests) has been induced in horses that have had their thyroid gland removed (thyroidectomy). The condition has also been induced in horses through the oral administration of the antithyroid medication propylthiouracil (PTU). [1]

Secondary hypothyroidism describes the abnormal secretion of thyroid-stimulating hormone (TSH) from the anterior pituitary gland in the brain. [1]

Although not diagnosed in horses, tertiary hypothyroidism describes a condition in which an inadequate amount of thyrotropin-releasing hormone (TRH) is released from the hypothalamus. [1]

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