Hypothyroidism - Causes

A small percentage of Graves' disease patients who require surgery to remove most of both thyroid lobes (subtotal thyroidectomy) may develop hypothyroidism. It is important to find an experienced surgeon for this procedure and to have the thyroid checked at 6- or 12-month intervals.

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Thyroid Dysfunction Syndromes

Researchers have identified a number of additional syndromes that also cause hypothyroidism. These generally involve abnormalities in thyroid hormone itself or genetic deficiencies in certain proteins that impair thyroid hormone conversion processes or responses.

Drugs that Reduce Thyroid Levels

Lithium. Lithium, a drug widely used to treat psychiatric disorders, has multiple effects on thyroid hormone synthesis and secretion. Up to 50% of patients who take lithium develop a goiter, with 20% developing symptomatic hypothyroidism, and another 20 - 30% developing hypothyroidism without symptoms.

Amiodarone. The drug amiodarone (Cordarone), which is used to treat abnormal heart rhythms, contains high levels of iodine and can induce hyper- or hypothyroidism, particularly in patients with existing thyroid problems. Hypothyroidism occurs in 20% of patients and is the more common effect in the US and other countries where dietary iodine is abundant. Hyperthyroidism is a less common effect in these regions.

Other Drugs. Drugs used for epilepsy, including phenytoin and carbamazepine, reduce thyroid levels. Certain antidepressants may cause hypothyroidism, although this is rare. Interferons and interleukins are used in hepatitis, multiple sclerosis, and other conditions. Evidence suggests that these drugs increase antibodies that put patients at risk for hypo- or hyperthyroidism. Many other medications contain iodine or have properties that affect the thyroid, although their effects are almost always reversible when they are stopped. Large doses of selenium, a common over-the-counter supplement, may lower thyroid levels. Some drugs used in cancer chemotherapy can also cause hypothyroidism.

Radiation

High-dose radiation for cancers of the head or neck and for Hodgkin's disease causes hypothyroidism in up to 65% of patients within 10 years after treatment.

Causes of Secondary and Tertiary Hypothyroidism

In rare instances, usually due to a tumor, the pituitary gland will fail to produce thyrotropin (TSH), the hormone that stimulates the thyroid to produce its hormones. In such cases, the thyroid gland withers. When this happens, secondary hypothyroidism occurs.

Causes of Hypothyroidism in Infants

Hypothyroidism in newborns (known as congenital hypothyroidism) occurs in one in every 3,000 to 4,000 births, making it the most common hormonal disorder in infants. In 90% of these cases, it persists throughout life.