Medical Edge Newspaper Column

What Causes Hypothyroidism?

November 11, 2007
Dear Mayo Clinic:
What causes hypothyroidism? Is it inherited? Is it the result of a nutritional deficiency?

Answer:
Hypothyroidism is sometimes referred to as "underactive thyroid." Your thyroid is a small gland at the base of your neck that has a big impact on how your body works. The hormones your thyroid produces help regulate your metabolism, including how efficiently you burn calories and maintain your body temperature and heart rate. These hormones also affect your skin, muscle strength, appetite and mood. In fact, how well your body functions overall depends on your thyroid hormones.

When your thyroid doesn't make enough of two hormones — triiodothyronine (T-3) and thyroxine (T-4) — hypothyroidism results and your metabolism slows. This can lead to sluggishness, weight gain, dry skin and hair, a hoarse voice, and increased sensitivity to cold. Left untreated, hypothyroidism can lead to potentially serious complications, including high cholesterol levels and heart disease, such as congestive heart failure.

The thyroid's production of T-3 and T-4 is governed by two other parts of your body: the pituitary gland and the hypothalamus. Your pituitary gland is the master gland that controls all the other glands in your body's hormone-producing (endocrine) system. Through its production of thyroid stimulating hormone (TSH), the pituitary tells the thyroid how much T-3 and T-4 to make. One of the jobs of the hypothalamus — the part of your brain that controls the entire endocrine system — is to signal to the pituitary gland how much TSH to make by secreting another hormone: thyrotropin-releasing hormone (TRH).

Although a defect anywhere in the hypothalamus, pituitary or thyroid can lead to hypothyroidism, most problems that trigger the condition directly affect the thyroid. It's rare for a decrease of TSH from the pituitary gland or decreased secretion of TRH from the hypothalamus to cause hypothyroidism.

In regard to your question about whether hypothyroidism is inherited, thyroid disorders do tend to run in families, but the inherited form of hypothyroidism is rare.

As to nutritional deficiency, a lack of the mineral iodine can play a part in hypothyroidism. In fact, worldwide, a lack of iodine is one of the most common causes of the disease. The body needs iodine to produce T-3 and T-4. In the United States, however, iodine deficiency is rare, mainly due to the addition of iodine to table salt.

Hypothyroidism in this country is more commonly caused by autoimmune thyroiditis (Hashimoto's thyroid disease). In this disorder, your body's immune system produces antibodies that attack tissue in the thyroid, affecting the gland's ability to produce hormones.

Radiation therapy or thyroid surgery for cancer or other thyroid disease may also decrease thyroid function. Hypothyroidism can be a side effect of medications such as thionamides, lithium, amiodarone, interferon-alpha, interleukin-2 and perchlorate. In some cases, treatment for hyperthyroidism (overactive thyroid) may reduce thyroid function too much, and hypothyroidism can result.

In addition, underlying medical conditions can also cause hypothyroidism. These include the liver disorder hemochromatosis and sarcoidosis. In some women, pregnancy triggers an inflammation of the thyroid (postpartum thyroiditis), which can lead to hypothyroidism.

No matter what the cause, hypothyroidism usually can be effectively treated by replacing the missing hormone with a synthetic form of thyroid hormone.

— Diana Dean, M.D., Endocrinology, Mayo Clinic, Rochester, Minn.