Hyperthyroidism
Basic Information 
• The equine thyroid glands are two firm lobes located in the dorsolateral aspect of the third to sixth tracheal rings.
• In most healthy horses, the thyroid glands are not visible but are palpable.
• The follicular cells of the thyroid gland secrete two thyroid hormones (THs), thyroxine (T4) and triiodothyronine (T3).
• The synthesis and secretion of THs is regulated by a negative feedback system that includes the hypothalamus, pituitary gland, and thyroid gland (hypothalamus-pituitary-thyroid axis)
• Hypothalamic neurons release thyrotropin-releasing hormone (TRH), which stimulates thyrotropes in the pituitary gland to secrete thyroid-stimulating hormone (TSH, thyrotropin), which in turn increases the synthesis and secretion of THs by the follicular cells of the thyroid gland.
• Most of the THs released into circulation is T4, which is converted to active T3 by deiodinases.
• Circulating THs are T4, T3, and reverse T3 (rT3).
• Free T3 is the most active hormone.
• THs circulate free and bound to proteins.
• THs are important in cell growth, differentiation, and energy metabolism.
• An increase in THs leads to a hypermetabolic state; a decrease results in the opposite.
Clinical Presentation
History, Chief Complaint
• Horses with hyperthyroidism have a history of hyperexcitability, weight loss, polyphagia, and polydipsia.
• Thyroid gland enlargement may be present, although this rarely occurs.
• Anhidrosis and hyperhidrosis have been reported.
• Hyperthyroidism has not been documented in foals.
• In adult horses, there is a history of weight loss.
• There may be unilateral thyroid enlargement.
• Hyperexcitability, tachycardia, tachypnea, tremors, sweating, alopecia, polydipsia, anhidrosis, hyperhidrosis, enophthalmos, and weight loss have been described in horses suspected of hyperthyroidism.
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
