DEAR DR. ROACH: I know an elderly woman who has been taking 60 milligrams of NP thyroid, then she needed a new doctor, who prescribed 37.5 micrograms of levothyroxine. Why would the new doctor change her thyroid medication when she is having no problems with the NP? Ever since the change, she suffers from headaches in the left rear side of her head. Is this common practice for new doctors to change medications? — T.H.

ANSWER: NP Thyroid, like Armour thyroid, is a natural product made from desiccated animal thyroid glands. It contains both thyroxine (also called T4) and triiodothyronine (also called T3). T3 is the active form of the hormone. The vast majority of people taking thyroid hormone replacement take only T4, which the body converts to T3.

Many physicians avoid using natural products because the ratio of T3 to T4 in the animal thyroid gland (roughly 4:1) is not what is naturally found in humans. It’s normally about 15:1. T3 does not last long in the system, and a person taking desiccated thyroid hormone may have higher than normal T3 levels for a few hours after taking the medication and lower than normal levels right before taking the next dose. Also, animal products naturally have variability from one batch to another, despite the efforts by the manufacturers to ensure consistency. Using synthetic levothyroxine allows much more consistent dosing. Most guidelines advise switching to synthetic.

The usual conversion of 60 mg desiccated thyroid extract to T4 is 75-100 micrograms of T4. It sounds like the elderly woman you are asking about had a big drop, more than 50%, in her effective thyroid replacement dose. I don’t know what her blood hormone levels were, so this may have been intentional. If not, that might explain the symptoms.

About 10% of people who switch from natural do not feel as well on synthetic hormones. In this case, some physicians will try adding in pharmaceutical-grade T3, keeping the proper ratio of T4 to T3 for humans. There are some people (about 16%) who have a different type of an enzyme called type 2 deiodinase, and those people are not well able to convert T4 to the more active T3. It may be, but isn’t proven, that people who can’t convert T4 to T3 so well would benefit from additional T3, but I would recommend doing so with as much control as possible, using exactly calibrated doses of T4 and T3. T3 needs to be dosed twice daily.

I have patients come to me on desiccated thyroid preparations occasionally. If they have no symptoms and their thyroid test results are normal, I don’t always change them. However, the possibility of fluctuating T3 levels is a good reason to switch.

DEAR DR. ROACH: I read in my morning newspaper that you recommend two doses of the vaccine to prevent shingles. Years ago, I received one dose, although I forget the name of it. What exactly is the second dose? Right now, I’m 70 years old and do not have shingles. — B.B.

ANSWER: If you had a single dose years ago, you had Zostavax, a live vaccine that afforded some protection, but the protection wasn’t perfect and started to wear off after a few years. The new vaccine is called Shingrix, and is a two-dose series. Two doses are necessary even if you had the previous Zostavax, and Shingrix is recommended if you are over 50, whether or not you have had the previous vaccine, and whether or not you have already had shingles.

If you actively have shingles, it’s recommended you wait until the shingles is completely gone before getting the vaccine.

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Dr. Roach regrets that he is unable to answer individual letters, but will incorporate them in the column whenever possible. Readers may email questions to ToYourGoodHealth@med.cornell.edu or send mail to 628 Virginia Dr., Orlando, FL 32803.