Hypothyroidism - Causes

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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.

Permanent Congenital Hypothyroidism. In up to 85% of permanent congenital hypothyroidism cases, the thyroid gland is missing, underdeveloped, or not properly located. In most cases the cause or causes of these conditions are unknown. In about 10 - 15% of cases, processes involved in hormone production are impaired, most likely because of genetic abnormalities. In less than 5% of cases, the pituitary or hypothalamus function abnormally.

Temporary Hypothyroidism in Infants. Temporary hypothyroidism can also occur in infants. In about 20% of cases, the cause remains unknown. The known causes stem from various immunologic, environmental, and genetic factors, including those in the mother:

• Thyroid dysfunction in the mother during pregnancy. Hypothyroidism during pregnancy and anti-thyroid medications in pregnant hyperthyroid women are major causes of temporary hypothyroidism in infants. All mothers with hypothyroidism should be treated effectively. Their babies' risk for thyroid dysfunction can persist if therapy is inadequate.
• Iodine deficiency. This may cause temporary hypothyroidism. (Exposure to too much iodine immediately after birth, for example, from iodine-containing disinfectants or medicines, can also cause thyroid dysfunction.)
• Being premature.
• Kidney disease. Temporary hypothyroidism in infants can occur in premature babies and, rarely, in those with kidney disease.
• The central nervous system connections between the hypothalamus and pituitary gland may also mature late; this condition generally resolves 4 to 16 weeks after birth.

Children with temporary congenital hypothyroidism should be followed-up regularly during adolescence and adulthood for possible thyroid problems. The risk for further thyroid problems is highest in these adult women during pregnancy. Newborn siblings of these children should also be screened for possible thyroid defects.

Review Date: 04/19/2006
Reviewed By: Harvey Simon, M.D., Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital