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Ask Dr. K: Depression and hypothyroidism: Is there a connection?

DEAR DOCTOR K: For years I took antidepressants, but they didn’t really help my depression. Then a new doctor diagnosed me with hypothyroidism. Once I started taking thyroid medication, the depression lifted. What’s the connection?

DEAR READER: Hypothyroidism, or an underactive thyroid gland, is one cause of depression. Depression is a condition caused by chemical changes in the brain. Surely life experiences can affect those chemical changes, making them worse or better. But at its root, depression involves brain chemistry.

The chemistry of the brain is also influenced by the chemistry of the blood, since blood circulates throughout the brain. The thyroid gland, a small, butterfly-shaped structure that sits low in the neck, secretes hormones that regulate the body’s energy. In this way, the thyroid indirectly affects everything from muscles, bones and skin, to the digestive tract, heart and brain.

The thyroid, in turn, is regulated by the pituitary gland. This pea-sized gland hangs down from the bottom part of the brain. When blood levels of thyroid hormones fall, the pituitary gland sends signals to the thyroid, telling it to get to work. In response, the thyroid produces two hormones, T3 and T4. If you have hypothyroidism, however, your thyroid gland does not produce enough thyroid hormones. Blood levels of T3 and T4 remain low.

When thyroid hormone levels are low, organs and internal systems slow down, creating a wide range of symptoms. Many of these symptoms — depressed mood, fatigue, weight gain, reduced sexual desire and trouble concentrating — are also symptoms of depression. Not surprisingly, hypothyroidism is often misdiagnosed as depression. Here’s an example:

My wife and I are friends with a couple who about 20 years ago started to have marital troubles. She was moody. Her husband thought she was depressed, and her doctor agreed. They separated.

One winter evening she slipped on the ice and broke her ankle. In the emergency room, the orthopedic surgeon took one look at her and asked, “Has your doctor ever said you have an underactive thyroid?” He did the test, confirmed his suspicions, and she got the treatment she needed. The couple reunited a few months later.

Other symptoms are more typical of either depression or hypothyroidism and can help tell the two conditions apart. (I’ve put a table listing symptoms of each condition, along with symptoms the two conditions share, on my website, AskDoctorK.com.)

Hypothyroidism can be confirmed or ruled out with a blood test. Once diagnosed, it is usually treated with a thyroid hormone pill that restores thyroid hormone levels to normal. As in your case, depression usually subsides once you start taking this medication.

Researchers aren’t entirely sure why there is a link between hypothyroidism and depression. Clearly the low thyroid hormone levels in the blood affect the chemistry in the brain that leads to depression. In fact, doctors sometimes add thyroid medications to antidepressant treatment to improve mood — even in patients who have normal thyroid function.

Dr. Komaroff is a physician and professor at Harvard Medical School. To send questions, go to AskDoctorK.com, or write: Ask Doctor K, 10 Shattuck St., Second Floor, Boston, MA 02115.

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