DEAR MAYO CLINIC: I'm wondering if I have hypothyroidism. My thyroid-stimulating hormone (TSH) is normal, but my free thyroxine is on the low end of the normal range. My doctor recommends I take a small dosage of a synthetic thyroid hormone. Do I need to take this medication, and if so, for how long? What are the dangers associated with hypothyroidism?

ANSWER: Hypothyroidism -- an underactive thyroid -- is a common and treatable disorder. It's especially common in women over age 50. However, it can occur in both men and women at any age, even in babies.

Your test results don't seem to indicate hypothyroidism.

Here's some background that will help explain why you may need to search out a different diagnosis for any symptoms that led up to the test results you've shared.

The thyroid is the butterfly-shaped gland at the base of the front of the neck. Its job is to produce two main hormones, triiodothyronine and thyroxine. Both of these hormones are critical to all aspects of the body's metabolism. Among their functions, they regulate body temperature and heart rate. And they determine the rate in which you use fats and carbohydrates.

When your thyroid isn't producing enough of these hormones and the condition remains untreated, the long-term consequences can include obesity, joint pain, depression, infertility and heart disease.

Early symptoms of hypothyroidism vary widely and often develop slowly -- over years. Possible symptoms include sluggishness, fatigue, constipation, dry skin, increased sensitivity to cold, unexplained weight gain and elevated cholesterol levels. I'm guessing that you experienced some of these early symptoms, which resulted in your physician ordering tests for thyroid function. That sequence of events makes perfect sense.

The TSH test, in particular, is critical to a diagnosis of hypothyroidism. This blood test is specific and reliable. Once symptoms are present, a TSH test result is the first measurable indication that something is amiss with the thyroid.

TSH, produced and released by the pituitary gland, signals the thyroid to produce its hormones. With an underperforming thyroid, the TSH reading would be high -- the result of the pituitary's effort to stimulate the thyroid gland to do its job.

Since your TSH test was normal, it's unlikely you have hypothyroidism. Your test result for free thyroxine, still in the normal range, doesn't indicate a concern either. The thyroxine level is unlikely to be abnormal unless the TSH is abnormal. These normal lab results don't seem to support the need for a synthetically-made thyroid hormone.

People with hypothyroidism benefit from a synthetically-made thyroid hormone. It's needed for the rest of their lives. Once the correct dose is established, the treatment is considered simple, safe and effective.

There's one caveat with this conclusion. I'm assuming that your pituitary gland is working properly. While pituitary problems are rare, the use of TSH as the primary test of thyroid function assumes that the pituitary function is normal. Your physician should at least consider the possibility of pituitary dysfunction as a cause of the low normal thyroxine and normal TSH results. But without other suggestive symptoms, formal testing of the pituitary may not be needed or helpful.

You didn't share what symptoms caused you to seek care. If concerns were broad and general, such as fatigue, sluggishness or constipation, it's reasonable to look at the thyroid as a possible culprit. But those broad symptoms could indicate a hundred different things. It's possible to have many of the symptoms of hypothyroidism and have a perfectly functioning thyroid.

I'd suggest revisiting a physician with a fresh look at your symptoms. You can probably cross off hypothyroidism as the underlying cause.

-- John Morris, M.D., Endocrinology, Mayo Clinic, Rochester, Minn.

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