Hypothyroidism is the most common type of thyroid disorder in cattle and is caused by defective thyroglobulin synthesis and secretion. This disease is rarely seen nowadays with feed supplements containing adequate dietary sources of iodine. Most cases arise as a congenital disease, developing as an idiopathic atrophy of the thyroid gland. There is some evidence that flouridation of cattle drinking water may contribute to this disease. Lymphocytic thyroiditis has also been reported in cattle.
Hypothyroidism is rarely diagnosed in younger cattle except as an incidental finding. In clinically affected cattle, signs include recurrent hoof overgrowth, persistent anestrous, obesity, dull hair coat associated with symmetrical alopecia, and decreased activity level.
Induced hypothyroidism in superovulated Brahman cows showed markedly reduced milk production but improved weight gain, body condition scores and fertility. Some cattle, however, still display signs of oestrus after thyroidecty, suggesting oestrus is not solely regulated by thyrotropic mechanisms.
Diagnosis is usually based on presenting clinical signs supported by laboratory demonstration of elevated levels of total triiodothyronine and total thyroxine.
Treatment is usually successful with oral levothyroxine and monitoring improvement in clinical signs included weight loss, hair regrowth, and reproductive cycling.
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