Medicine Cabinet

Q: I have an underactive thyroid and ran out of my thyroid medication two months ago. I just had a blood test and my doctor tells me my TSH is very high at 59. I feel fine. Do I really need to restart Levoxyl?

A: TSH, short for thyroid-stimulating hormone, is a small protein secreted by the pituitary gland in the brain that encourages the thyroid gland to make more thyroid hormone.

Normally, the TSH is less than 5. When the thyroid gland is not functioning properly, however, the pituitary gland secretes more and more TSH to try to make the thyroid respond and work harder. An elevated level of TSH confirms a diagnosis of an underactive thyroid (hypothyroidism).

Even though you say you feel fine now, you will almost surely develop symptoms within the next month or two. The most common symptoms of an underactive thyroid include low energy level, muscle aches, dry skin and intolerance to cold temperatures.

I encourage you to restart thyroid medication now. I suspect that you will feel more energetic once your TSH level returns to a normal range.

Q: I take the generic form of Levoxyl for hypothyroidism. When I tried to renew the generic, my doctor refused. She says that she only uses name brand. Is there really such a difference in the effectiveness of the generic in this case?

A: Levoxyl is one brand of levothyroxine, the most commonly used preparation to treat a person with an underactive thyroid gland. Originally there was only one brand of levothyroxine, Synthroid. Levoxyl was actually the first generic version of the drug.

Several companies now make levothyroxine approved by the U.S. Food and Drug Administration (FDA).

I have a theory as to why your doctor might be concerned. Although FDA generics are very reliable, the coating and other features of each generic may result in different intestinal absorption patterns even in the same person.

The dose of thyroid is regulated to keep the blood level of thyroid stimulating hormone in the normal range. If you are switching from one generic manufacturer to another and then back again, this can make it more difficult for your doctor to keep you in thyroid balance.

Usually the price difference between levo-thyroxine generics is small. Check with your pharmacist.

If there is a big difference in price, let your doctor know. Your doctor may be more comfortable as long as he or she knows that your pharmacy won’t be switching generic manufacturers every few months.

This column was written by Howard LeWine, M.D., a member of the Harvard Medical School faculty and practicing internist with Harvard Vanguard Medical Associates and Brigham and Women’s Hospital, Boston, Mass. He serves as chief medical editor of Internet Publishing at Harvard Health Publications. For additional consumer health information, visit health.harvard.edu.