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From SNPedia

Hypothyroidism, also called underactive thyroid or low thyroid, is a common disorder of the endocrine system. It can cause a number of symptoms, such as poor ability to tolerate cold, a feeling of tiredness, constipation, depression, and weight gain. (Wikipedia)

Thyroid hormone is required for the normal functioning of numerous tissues in the body. In health, the thyroid gland predominantly secretes thyroxine (T4), which is converted into triiodothyronine (T3) in other organs by the selenium-dependent enzyme iodothyronine deiodinase. Triiodothyronine binds to the thyroid hormone receptor in the nucleus of cells, where it stimulates the turning on of particular genes and the production of specific proteins. (Wikipedia)

In Western countries, hypothyroidism occurs in 0.3–0.4% of people. Subclinical hypothyroidism, a milder form of hypothyroidism characterized by normal thyroxine levels and an elevated TSH level, is thought to occur in 4.3–8.5% of people. Hypothyroidism is more common in women than men, and people over the age of 60 are more commonly affected. (Wikipedia)

A common cause of hypothyroidism is Autoimmune thyroiditis or autoimmune thyroid disease (AITD), the two main subtypes of which are Hashimoto's thyroiditis (HT) and Graves' disease (GD). Many genes have associations with AITD in general, and some are more associated with HT than GD.

In addition to being caused by a damaged or dysfunctional thyroid gland, hypothyroidism may also be caused by dysregulation among the three DIO (deiodinase) genes, DIO1, DIO2, and DIO3, that produce proteins that metabolize and catabolize thyroid hormones throughout the body and assist in managing the hypothalamic–pituitary–thyroid axis.