Causes

Many permanent or temporary conditions can reduce thyroid hormone secretion and cause hypothyroidism. About 95% of hypothyroidism cases occur from problems that originate in the thyroid gland. In such cases, the disorder is called primary hypothyroidism. (Secondary hypothyroidism is caused by disorders of the pituitary gland. Tertiary hypothyroidism is caused by disorders of the hypothalamus.)

The two most common known causes of primary hypothyroidism are the following:

• Hashimoto's thyroiditis. This is an autoimmune condition in which the body's immune system attacks its own cells.
• Overtreatment of hyperthyroidism (an overactive thyroid).

Thyroid Autoimmunity (Hashimoto's Thyroiditis and Others)

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Hashimoto's thyroiditis, atrophic thyroiditis, and postpartum thyroiditis are all autoimmune diseases of the thyroid. An autoimmune disease occurs when the immune system mistakenly attacks the body's own healthy cells. In the case of autoimmune thyroiditis, a common form of primary hypothyroid disease, the cells under attack are in the thyroid gland.

All forms of thyroid autoimmunity typically start with the following:

• Important immune factors called T and B cells infiltrate the thyroid gland in equal numbers. These white blood cells are the primary infection-fighting immune cells. T cells identify invasive molecules, such as viral proteins, and help B cells to produce antibodies that are designed specifically to attack these invaders.
• In cases of autoimmunity, T cells are tricked into classifying molecules on the body's own cells as invaders. In such cases, B cells then produce antibodies, called autoantibodies, which attack those cells.
• In most cases of thyroid autoimmunity, the autoantibodies launch an attack on a thyroid protein called thyroid peroxidase; this attack appears to destroy thyroid cells.