• Mayo Clinic Q and A: Should I have my TSH level rechecked?

a medical illustration of hypothyroidismDEAR MAYO CLINIC: I started taking levothyroxine more than five years ago for hypothyroidism. I had my TSH level tested about six months after I began taking it but have not had it checked since. I recently read a study saying this medication is often prescribed even when it’s not necessary. Should I see my doctor to be retested?

ANSWER: Yes. Make an appointment to have your condition re-evaluated at this time. For some people, lifelong treatment of hypothyroidism with the drug levothyroxine is necessary. But studies have found that for many others who have elevated levels of thyroid-stimulating hormone, or TSH, the medication isn’t needed. In fact, if it’s taken incorrectly or in doses that are too large, levothyroxine can cause harmful side effects.

Your thyroid is a small, butterfly-shaped gland at the base of the front of your neck. The thyroid gland makes two hormones — triiodothyronine, or T3, and thyroxine, or T4 — that have a large impact on your health, affecting all aspects of your metabolism. They maintain the rate at which your body uses fats and carbohydrates, help control your body temperature, influence your heart rate, and help regulate the production of proteins. The rate at which your thyroid makes T3 and T4 is regulated by another hormone that your pituitary gland produces, called TSH.

Hypothyroidism, sometimes called underactive thyroid, is a condition in which your thyroid gland doesn't produce enough T3 or T4. When blood tests show that you have high levels of TSH and low levels of T3 or T4 in your body, then a diagnosis of hypothyroidism is clear. Treatment with levothyroxine — a synthetic version of thyroid hormones — is necessary in almost all cases. But hypothyroidism is rare, affecting only about 0.2 percent of the population.

Much more common, affecting about 12 percent of the population, is a condition known as subclinical hypothyroidism. With this condition, your TSH level is above normal, but T3 and T4 levels are normal.

If a blood test shows you have subclinical hypothyroidism, and you don’t have any symptoms — such as fluid retention, fatigue, increased sensitivity to cold, constipation, muscle weakness or painful joints — treatment typically is not recommended. There are a few reasons for that.

First, about 30 percent of people whose condition falls into the category of subclinical hypothyroidism have their TSH levels return to normal within one year without treatment. Only 3 percent per year go on to develop hypothyroidism. Second, if you take too much levothyroxine or if you don’t take it correctly, it can negatively affect a variety of your body’s systems, including your brain, heart and muscle function. It also can interfere with how your body handles fluid and fats.

If, as in your case, you are receiving treatment for hypothyroidism, it’s important to have regular checkups. Testing TSH is one way to see if treatment is working. It’s also important for your health care provider to check your T4 levels.

Talk with your health care provider about the goals of treatment, too. If you started taking levothyroxine to control symptoms, make sure that you are seeing some benefit. Also, keep in mind that the symptoms of hypothyroidism often can be vague. If your symptoms don’t go away when you’re taking thyroid medication, it’s possible those symptoms could be linked to another medical condition.

Getting your TSH and T4 levels checked and reviewing any symptoms you may have with your health care provider should help clarify whether you need to continue taking levothyroxine. Dr. Juan Brito Campana, Endocrinology, Mayo Clinic, Rochester, Minnesota

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