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

Drug-induced hypothyroidism may be caused by:

• Drugs used for hyperthyroidism (overactive thyroid), such as propylthiouracil (PTU), radioactive iodine, potassium iodide, and methimazole
• Treatment with lithium or iodides in certain people

Unusual causes of drug-induced hypothyroidism include:

• Amiodarone
• Eating a large amount of iodine-containing seaweed
• Nitroprusside
• Perchlorate
• Povidone iodine (Betadine)
• Sulfonylureas

Too much iodine may cause either hyperthyroidism or hypothyroidism. For example, iodine used by the mother during pregnancy (or as an antiseptic in the delivery room) can cause the fetus or newborn to develop goiter (enlarged thyroid gland). This can cause temporary hypothyroidism.

The most severe form of hypothyroidism is called myxedema coma. This is a medical emergency. Severe hypothyroidism, however, is rarely related to drugs.

Early symptoms:

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

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

Joint stiffness may also occur.

A physical examination may reveal an enlarged thyroid gland. Other signs may include:

• Low blood pressure
• Low temperature
• Slow heart rate

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)

Other abnormalities that may be discovered in the laboratory include:

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

Stop taking the drug causing the hypothyroidism, if possible. However, do not stop taking prescribed medications without first talking to your healthcare provider. Some drugs may cause unpleasant or even life-threatening reactions if not stopped gradually or replaced.

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 you have started replacement therapy, report symptoms of increased thyroid activity (hyperthyroidism):

• Rapid weight loss
• Restlessness
• Sweating

If a period of lowered thyroid activity has led to weight gain, a high-fiber, low-calorie diet and moderate activity can help relieve constipation and promote weight loss.

With early treatment, your condition should return to normal. 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:

• Below-normal body temperature
• Decreased breathing
• Low blood pressure
• Low blood sugar
• Unresponsiveness to stimulation (decreased consciousness)

Other complications of hypothyroidism are:

• Heart disease
• Infertility
• Miscarriage in pregnant women
• Pituitary tumors (extremely rare)

Call your health care provider if you have 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.