Drug-induced hypothyroidism is an underactive thyroid gland due to a reaction from medication. "Drug-induced" means caused or brought on by medication.

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 people who may be predisposed to such reactions. Drugs used for hyperthyroidism (overactive thyroid), such as propylthiouracil (PTU), radioactive iodine, potassium iodide, and methimazole, may cause hypothyroidism. Unusual causes of drug-induced hypothyroidism include nitroprusside, perchlorate, amiodarone, povidone iodine (Betadine), sulfonylureas, or even ingesting a large amount of iodine-containing seaweed.

Note that too much iodine may cause either hyperthyroidism or hypothyroidism. For example, iodine used by the mother during pregnancy (or even when used as an antiseptic in the delivery room) can cause the fetus or newborn to develop goiter (enlarged thyroid gland), and at least temporary hypothyroidism.

The most severe form of hypothyroidism is called myxedema coma, which is a medical emergency. Severe hypothyroidism, however, is seldom drug-related.

Early symptoms:

• Weakness
• Fatigue
• Cold intolerance
• Constipation
• Weight gain
• Depression
• Brittle fingernails
• Coarsening and thinning of hair
• Dry skin
• Puffy eyes

• Slow speech
• Thickening of the skin
• Puffy face, hands, and feet
• Decreased hearing ability
• Thinning of eyebrows
• Hoarseness
• Menstrual disorders

A physical examination may reveal an enlarged thyroid gland. Your 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 show an enlarged heart.

Blood tests to determine thyroid function include:

• Free T4 test
• T3 levels
• TSH (thyroid stimulating hormone)

• Increased cholesterol
• Increased liver enzymes
• Increased serum prolactin
• Low serum sodium
• Low blood glucose
• CBC that shows anemia

The drug causing the hypothyroidism must 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 sopped gradually and slowly, or replaced appropriately.

Levothyroxine, a thyroid replacement hormone, is the most commonly used medication to treat this condition. The dose is adjusted to bring TSH to normal levels. 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 period of lowered thyroid activity has led to weight gain.

With early treatment, return to the normal state is usual. However, hypothyroidism will return if the replacement therapy is not continued. Myxedema coma can result in death.

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 of hypothyroidism are heart disease, infertility, miscarriage in pregnant women, and pituitary tumors (extremely rare).

Call your health care provider if you have signs or symptoms of hypothyroidism.

Medications that can cause hypothyroidism should be used with caution. If you take such medicines, you are usually monitored closely. For example, your thyroid levels may be checked every once in a while.