Drug-induced hypothyroidism involves decreased activity of the thyroid gland caused by an adverse reaction to a medication.

Causes, incidence, and risk factors

Hypothyroidism (underactivity of the thyroid gland) can cause a variety of symptoms that affect the entire body. The body's normal rate of functioning slows, causing mental and physical sluggishness. Symptoms vary from mild to severe. Drug-induced hypothyroidism may be caused by treatment with lithium or iodides in susceptible people. Drugs used for hyperthyroidism (overactive thyroid) such as PTU and methimazole may cause unintentional hypothyroidism. Unusual causes of drug-induced hypothyroidism include nitroprusside, perchlorate, and sulfonylureas. The most severe form of hypothyroidism is called myxedema coma , which is a medical emergency. Severe hypothyroidism, however, is seldom drug-related.

Signs and tests

A physical examination may reveal an enlarged thyroid gland and delayed deep tendon reflexes. An assessment of vital signs (temperature, pulse , rate of breathing, blood pressure ) may reveal slow heart rate , low blood pressure , and low temperature. A chest X-ray may reveal an enlarged heart. Laboratory tests to determine thyroid function include the following:

Treatment

The drug causing the hypothyroidism may be discontinued if possible. However, do not stop taking prescribed medications without first consulting your healthcare provider, as some may cause unpleasant or even life-threatening reactions if not tapered slowly or replaced appropriately. Levothyroxine, a thyroid replacement hormone, is the most commonly used medication to treat this condition. The dose is adjusted to normalize TSH. After replacement therapy has begun, report symptoms of increased thyroid activity ( hyperthyroidism ) -- restlessness , rapid weight loss or sweating if they occur. A high-fiber , low-calorie diet and moderate activity can help relieve constipation and promote weight loss if a peroid of lowered thyroid activity has lead to weight gain.

Expectations (prognosis)

With early treatment, return to the normal state is usual. However, relapses will occur if the medication is not continued. Myxedema coma can result in death.

Complications

Myxedema coma, the most severe form of hypothyroidism , is rare. It can be caused by an infection, illness, exposure to cold, or certain medications. Symptoms and signs of myxedema coma include unresponsiveness to stimulation (decreased consciousness), decreased breathing, low blood pressure , low blood sugar , and below-normal body temperature. Other complications are heart disease , infertility , miscarriage in pregnant women, and pituitary tumors (extremely rare).

Calling your health care provider

Call your health care provider if signs of myxedema are present; or if chest pain , rapid heartbeat, or infection occur.

Prevention

Medications that can cause hypothyroidism should be used with caution. Usually the patient is monitored closely when on these medications. For example, thyroid levels may be checked periodically.