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Hypothyroidism is the disease state in humans and animals caused by insufficient production of thyroid hormone by the thyroid gland. There are several distinct causes for chronic hypothyroidism in human beings, the most common being Hashimoto’s thyroiditis (an autoimmune disease) and radioiodine therapy for hyperthyroidism. Historically, iodine deficiency was the most common cause of hypothyroidism world-wide. The disease may also be caused by a lack of thyroid gland or pituitary hormone.

Secondary hypothyroidism occurs if the pituitary gland is damaged by a tumour, radiation, or surgery so that it is no longer able to instruct the thyroid to make enough hormone. Hypothyroidism can also result from sporadic inheritance, sometimes autosomal recessive. It is a relatively common disease in purebred domestic dogs as well, and can have a hereditary basis in dogs.

The severity of hypothyroidism varies widely. Patients are classified as “subclinical hypothyroid” if diagnostic findings show thyroid hormone abnormalities, but they do not exhibit any symptoms. Others have moderate symptoms that can be mistaken for other diseases and states. Advanced hypothyroidism may cause severe complications, the most serious one of which is myxedema.

To diagnose the condition, levels of T4 (thyroxine) and TSH (thyroid stimulating hormone) are measured. Treatment comes in the form of thyroid hormone replacement therapy.

Signs and symptoms

Hypothyroidism has been called the “great imitator” for the vast number of medical conditions it can mimic. “Thyroid hormones are essential and primary regulators of the body’s metabolism. Imbalances can affect virtually every metabolic process in the body, exerting significant effects on mood and energy level. Thyroid function has a profound impact on overall health via its modulation of carbohydrate, protein, and fat metabolism, vitamin utilization, mitochondrial function, digestive process, muscle and nerve activity, blood flow, oxygen utilization, hormone secretion, sexual and reproductive health, and many other physiological parameters.