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The thyroid glands secrete hormones important for growth, maturation of organ systems and regulation of metabolism. These hormones stimulate protein synthesis and breakdown, increase body heat production and stimulate basal metabolic rate. Thyroid hormones circulate in both a protein-bound form and an unbound or free form, the latter of which is the active portion. Triiodothyronine (T3) and thyroxine (T4) are both active, with T3 being more metabolically active than T4. Thyroid hormone secretion is regulated by the hypothalamus and pituitary. Abnormal thyroid hormonal states described in horses include thyroid gland neoplasia, hyperthyroidism and hypothyroidism. Certain drugs, diets and pathophysiologic states can influence circulating thyroid homrone concentrations. Thyroid Gland Neoplasia Thyroid gland neoplasia is not uncommon and is often found on necropsy as an incidental finding in older horses. Tumors reported include adenomas (most common), carcinomas, C-cell tumors and adenocarcinomas. Most thyroid gland tumors are benign and non-function; treatment is usually unnecessary unless the neoplasms are large enough to interfere with swallowing or breathing. Hyperthyroidism in Adult Horses Hyperthyroidism is extremely rare in horses and cases reported have associated this with thyroid gland neoplasia. Signs include weight loss, increased heart and respiratory rates, hyperactive behavior and weight loss despite a ravenous appetite. Diagnosis relies on measuring circulating concentrations of free thyroid hormones. Treatment relies on removal of the affected thyroid gland. If both glands require removal, thyroid supplementation may be necessary. Hypothyroidism in Adult Horses Hypothyroidism in adult horses is an area of controversy in equine medicine. Conditions that have been thought to be related to hypothyroidism include obesity, laminitis, anhidrosis (lack of sweating), recurrent tying up and poor fertility. However, proper documentation of hypothyroidism in most cases does not occur. As opposed to dogs and humans, autoimmune thyroid disease has not been described in horses. Clinical signs associated with hypothyroidism in horses include weight gain, cresty necks, recurrent laminitis; however these signs point more to equine metabolic syndrome than actual hypothyroidism. Horses that are made hypothyroid via removal of the thyroid glands do not exhibit changes commonly thought to be associated with hypothyroidism. Such horses are mainly lethargic, exercise intolerant, with a poor hair coat, decreased resting heart rate, cardiac output, respiratory rate and rectal temperature. Signs typically associated with hypothyroidism were absent. Certain drugs and diets are observed to change thyroid hormone synthesis, metabolism or binding and alter serum concentrations of thyroid hormones. Phenylbutazone or dexamethasone administration, strenuous exercise, diets high in energy, protein, zinc and copper have all been implicated. In addition, nonthyroidal illness syndrome has been described in other species in which there is a decrease in the level of free and bound thyroid hormones due to illness unassociated with the thyroid glands. Assessing thyroid function in adult horses relies on measuring free and total T4 and T3 and evaluating the response of these hormones to adminstration of TRH (thyroid releasing hormone) or TSH (thyroid stimulating hormone). It is important to remember to not measure hormonal levels for at least 2-4 weeks after the administration of medications that may interfere with thyroid hormone binding or after a severe illness. In addition, testing should be performed no sooner than 4 weeks after all thyroid hormone supplementation has been discontinued. TRH and TSH are not widely available commercially and require the collection of multiple samples of blood before and after administration. Measurement of the free fractions of thyroid hormones may be more useful than measuring total amounts. When a true case of adult hypothyroidism is diagnosed, treatment is straightforward and relies on the administration of thyroid hormones supplementation. However, it is important to monitor the levels of these hormones to ensure normalization of their levels. Thyroid Function in Foals Normal neonatal foals have serum concentrations of thyroid hormones 10 times the concentrations of adult levels, with these declining and approaching adult levels by one month of age. Premature foals have lower serum concentrations of total and free thyroid hormones than normal foals. In sick foals, it is thought that these experience nonthyroidal illness syndrome. Congenital hypothyroidism has been described in foals by mares ingesting too much or too little iodine and by mare ingesting goitrogenic plants. Foals born to these mares are weak, with a poor suckle reflex, poor righting reflexes, hypothermic, with tendon contracture or rupture and delayed bone development. Another syndrome observed in the western USA and Canada is characterized by thyroid gland hyperplasia, increased gestational length and musculoskeletal abnormalities.
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