Causes of Thyroid, under-active

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The commonest cause of hypothyroidism is an autoimmune inflammation of the thyroid gland (thyroiditis). This can result in a severely damaged thyroid gland that functions poorly or not at all. Hashimoto's thyroiditis is a common form of the disease which runs in families.

The second most common cause is medical overtreatment for hyperthyroidism (see article on Hyperthyroidism) with radioactive iodine or surgery. The damage to the thyroid gland may be permanent. Underactivity of the gland caused by drug treatment usually reverses after the treatment is complete.

Lack of enough iodine in the diet reduces the production of thyroid hormone. This causes the pituitary gland (a hormone-producing gland at the base of the skull) to produce more thyroid-stimulating hormone (TSH). The thyroid gland then works harder, becomes larger (this is visible as goitre) and traps all available iodine. Although this does not usually cause hypothyroidism, if the lack of iodine is very severe, hypothyroidism may result.

In Britain this cause of hypothyroidism is virtually unknown as even very poor diets contain enough iodine, but in some other countries there is the risk of iodine deficiency.

Hypothyroidism in children is usually caused by severe lack of iodine in the mother during the pregnancy and in the diet after birth. This has been common in countries where levels of iodine in the soil are low, but it is becoming less common since the cause has been understood and iodine has been added to domestic salt.