Hypothyroidism - Causes

(Page 4)

Drugs and Medical Treatments that Reduce Thyroid Levels

Lithium. Lithium, a drug widely used to treat bipolar disorder, has multiple effects on thyroid hormone synthesis and secretion. About 5 – 35% of patients treated with lithium go on to develop hypothyroidism and up to 50% of patients who take lithium develop a goiter. Most patients develop subclinical hypothyroidism, but a small percentage experience overt hypothyroidism.

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 about 20% of these patients and is the more common effect in the U.S. and other countries where dietary iodine is abundant. Hyperthyroidism is a less common effect in these regions.

---

---

Other Drugs. Drugs used for treating epilepsy, such as phenytoin and carbamazepine, can reduce thyroid levels. Certain antidepressants may cause hypothyroidism, although this is rare. Interferons and interleukins, which are used to treat hepatitis, multiple sclerosis, and other conditions, can also induce hypothyroidism or hyperthyroidism. Some drugs used in cancer chemotherapy, such as sunitinib (Sunent) or imatinib (Gleevec), can also cause or worsen hypothyroidism.

Radiation Therapy. High-dose radiation for cancers of the head or neck and for Hodgkin's disease can cause hypothyroidism up to 10 years after treatment.

Other Medical Conditions

Several medical conditions involve the thyroid and can change the normal gland tissue so that it no longer produces enough thyroid hormone. Examples include hemochromatosis, scleroderma, and amyloidosis.

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 shrinks. 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 - 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 cases of permanent congenital hypothyroidism, 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.