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JAMA Patient Page
July 9, 2019

Subclinical Hypothyroidism

JAMA. 2019;322(2):180. doi:10.1001/jama.2019.9508

Subclinical hypothyroidism is a condition that involves a slight decrease in the amount of thyroid hormone.

What Is Subclinical Hypothyroidism?

The thyroid gland is an organ in the front of the neck that produces a chemical called thyroid hormone. Thyroid hormone regulates the body’s metabolism (such as how fast things in the body work). If there is too much thyroid hormone (hyperthyroidism), a person may feel hot, lose weight, and have a rapid or irregular heart rate. When there is not enough thyroid hormone (hypothyroidism), a person may feel cold and tired and may gain weight. People with hypothyroidism tend to have high blood pressure and higher cholesterol levels, which are associated with an increased risk of heart attack and stroke.

In subclinical hypothyroidism, thyroid hormone levels in the blood are still in the normal range. But the levels are slightly lower than they should be for that particular person. In most cases, the condition is so mild that a person may not know they have it because none of the symptoms that usually occur with hypothyroidism are present.

How Does the Thyroid Work?

The pituitary gland at the base of the brain releases thyroid-stimulating hormone (TSH; also known as thyrotropin) into the blood. Thyroid-stimulating hormone travels to the thyroid gland and stimulates the thyroid to make and release thyroid hormone into the blood. Thyroid hormone then travels to the tissues throughout the body. If the thyroid gland is not functioning properly and thyroid hormone levels decline even slightly, the pituitary gland detects this and releases more TSH into the blood in an effort to stimulate the thyroid gland to make more thyroid hormone. However, because the thyroid is damaged or inflamed, it may not be able to respond to TSH, so the levels of thyroid hormone remain low. The higher the blood level of TSH, the worse the thyroid is functioning. A level of TSH between 5 and 10 mU/L is a mild elevation, and levels above 10 mU/L represent a more severe degree of thyroid malfunction.

Testing for Subclinical Hypothyroidism

Thyroid function tests are obtained when a patient reports having the symptoms described above, which suggests that there may be a thyroid problem. Also, thyroid hormone tests are obtained as part of routine medical care. If a person has an elevated blood level of TSH but the blood level of thyroid hormone is normal, he or she may have subclinical hypothyroidism.

Causes of Subclinical Hypothyroidism

The most common cause of subclinical hypothyroidism is a condition known as Hashimoto thyroiditis, a disease that involves inflammation and damage to the thyroid gland because of antibodies against a person’s own thyroid gland. A number of other conditions can cause subclinical hypothyroidism, including having had thyroid surgery in the past and having received radiation to the front of the neck. Subclinical hypothyroidism can also be caused by variety of drugs such as lithium, some drugs used to treat cancer, and amiodarone, a drug used to treat heart rhythm abnormalities.

Should You Be Treated for Subclinical Hypothyroidism?

There have not been large clinical trials answering the question of whether treatment for subclinical hypothyroidism improves a person’s health or life expectancy. Some experts recommend treating some patients with this condition. This is because there may be a benefit in correcting even slightly low thyroid hormone levels. However, blood TSH levels increase as people age, so having a slightly elevated TSH level after age 65 or 70 years is normal. People younger than about 65 years may benefit from treatment of subclinical hypothyroidism. Treatment involves taking a small amount of thyroid hormone replacement each day. It is generally agreed that if the TSH level is greater than 10 mU/L, thyroid hormone replacement should be given, including to patients older than 65 or 70 years.

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Conflict of Interest Disclosures: None reported.

Source: Biondi B, Cappola AR, Cooper DS. Subclinical hypothyroidism: a review [published July 9, 2019]. JAMA. doi:10.1001/jama.2019.9052