By Richard S. Haber, M.D.
Professor of Medicine, Division of Endocrinology
Icahn School of Medicine at Mount Sinai
Hypothyroidism, or an underactive thyroid gland, ranks among the most common hormonal disturbances. The thyroid hormone is secreted by the thyroid gland, which lies in front of and around the windpipe in the lower neck. It regulates energy usage in most tissues in the body. Major symptoms of thyroid hormone deficiency, which can be reversed with hormone replacement therapy, include fatigue and some weight gain as metabolism slows.
What Causes Hypothyroidism?
The most common cause is damage to the thyroid gland from autoimmune thyroid disease, in which the body's immune system attacks the thyroid over many years. This condition is also known as Hashimoto's thyroiditis, a chronic inflammation in the thyroid gland first recognized about 100 years ago. It is the most common of many autoimmune diseases in which the body's immune system attacks various tissues. As with other autoimmune diseases, hypothyroidism is more common in women and tends to run in families. Thyroid damage usually progresses very slowly, over years and decades, and the onset of symptoms may go unrecognized and undiagnosed.
Patients with autoimmune thyroid disease may also develop temporary hypothyroidism lasting for a few months after pregnancy. Autoimmune thyroid disease can also cause an overactive thyroid (hyperthyroidism or Graves' disease) in some patients.
Other common causes of hypothyroidism include surgical removal of the thyroid (usually for benign or malignant thyroid nodules) and radioactive iodine therapy (for overactive thyroid, hyperthyroidism). Less common causes of hypothyroidism include congenital hypothyroidism, transient hypothyroidism due to viral infections (subacute thyroiditis), and pituitary gland disease.
What Are the Symptoms of Hypothyroidism?
Most patients have some degree of fatigue, depending on the severity of the disturbance, and often experience modest weight gain. It is not a cause of severe obesity. Sleepiness during the daytime and a need for more hours of sleep is typical, as is a tendency to feel cold. In mild cases these symptoms may be subtle or absent.
Who Should be Tested?
- People with the symptoms listed above;
- People with family members with hypothyroidism or other forms of autoimmune thyroid disease; and
- People with other autoimmune diseases, such as rheumatoid arthritis, juvenile-onset diabetes, celiac disease, and lupus.
How is Hypothyroidism Diagnosed?
Although hypothyroidism is very common, its symptoms are non-specific: Fatigue and weight gain are common in people without thyroid disease. Therefore, the diagnosis depends on blood tests. Fortunately, these tests are simple, highly accurate, and able to diagnose even the mildest forms of hypothyroidism, before severe symptoms occur. The best test to screen for the disorder is TSH (thyroid stimulating hormone). This pituitary hormone, which stimulates and regulates the thyroid gland, rises when thyroid levels fall. This occurs even before thyroid hormone levels decline below the normal range, so the test is highly sensitive. Sometimes measurement of thyroid hormone in the blood (T4, free T4) is also included.
How is Hypothyroidism Treated?
Hypothyroidism is treated by thyroid hormone replacement therapy: The missing hormone is replaced as a daily supplement. For many years this was given in the form of tablets made from animal thyroid gland, but for the last 50 years pure thyroid hormone (levothyroxine) has been the preferred therapy. The dose is adjusted using the same TSH test used for diagnosis, and the goal is a normal TSH level. With an adequate dose, all symptoms of hypothyroidism are reversed and normal health is restored. Most patients take this treatment for life.