The Medicine Cabinet: Keeping an Eye on Thyroid Condition

The Medicine Cabinet: Keeping an Eye on Thyroid Condition

Q: I have been taking the same dose of thyroid medication for more than 30 years to treat an underactive thyroid gland. My last blood test was abnormal. Does that mean I need a higher dose of thyroid?

A: I suspect you had blood test to measure TSH, which stands for thyroid stimulating hormone, and the level was elevated. TSH is the test doctors periodically order when a person is being treated for hypothyroidism (an underactive thyroid gland) to see if the dose is correct. It’s also the initial test done when a person has symptoms that suggest a thyroid problem.

Thyroid stimulating hormone is made by the pituitary gland in the brain, and then released into the blood stream. When TSH reaches the thyroid gland, it stimulates the gland to produce thyroid hormone.

When the thyroid is making just the right amount of thyroxine hormone (T4), the blood TSH level will be in the normal range, 0.4 to 4.0 mU/L. However, many laboratories report a slightly different range and experts continue to debate what the normal range should be.

Here are some reasons why your TSH might be higher now.

Most people take a generic version of levo-thyroxine (Synthroid). You might be taking levo-thyroxine made by a different company than the pills you took before.

Have you started any new supplements or medications? Iron, other supplements and certain medications can interfere with absorption of levo-thyroxine. Check with your pharmacist about drug interactions.

Certain foods can lower the absorption of levo-thyroxine. Maybe you started taking your thyroids pills with meals. It’s best to take your pills one hour before eating or at bedtime.

TSH is a very sensitive indicator of thyroid function or whether the dose of replacement thyroid is correct. When the TSH is out of range, either too high or too low, the laboratory will also measure a thyroxine level (T4).

If your TSH is high and your T4 is low, then you will definitely need a higher dose of thyroid medication. But I suspect that your TSH is slightly high and your T4 is normal. If you are feeling fine, then you may need no dose adjustment if the TSH is below 7.0. If the level is higher than 7.0, doctors would most often recommend a small dose increase, even if you have no symptoms.

Howard LeWine, M.D. is an internist at Brigham and Women’s Hospital in Boston and assistant professor at Harvard Medical School.
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