To Your Health

My thyroid test results vary a lot and don't seem to match the way I feel. My prescription for thyroid is usually 75 micrograms (mcg) per day, but sometimes down to 50 mcg/day or up to 88 mcg/day. Why would it vary and what is the best way to tell if my thyroid is OK?

Q. My thyroid test results vary a lot and don't seem to match the way I feel. My prescription for thyroid is usually 75 micrograms (mcg) per day, but sometimes down to 50 mcg/day or up to 88 mcg/day. Why would it vary and what is the best way to tell if my thyroid is OK?

A. Thyroid status is one of the toughest puzzles in medicine, especially for women because there is a lot of interaction between female hormones and the thyroid. About 10 times as many women as men are diagnosed as hypothyroid. There are also several nutrients that are involved in making and using thyroid hormone, and variation in your nutritional status may be involved.

The thyroid gland makes the T4 thyroid hormone by attaching four iodine atoms to a pair of L-tyrosine molecules. The T4 form is also the form most often prescribed, as Synthroid. Over 99% of circulating thyroid hormone is in a sort of "time release" form and not immediately available to cells, or else we would see even more variation in the way we feel.

The amount you usually take, 75 mcg/day, is about half what the thyroid gland would usually produce. Hormones from the pituitary gland tightly regulate the amount of thyroid hormone you make, and the release of T4 into the bloodstream is also under the control of a pituitary hormone. From there it is taken up by body cells and converted within each cell to T3, the active form of the hormone, by removing one of the iodine atoms. This conversion uses an enzyme that needs selenium, and so if you are deficient in selenium you may have difficulty making that conversion.

Obviously iodine deficiency can result in low thyroid production and until iodized salt became common this was a major problem in certain parts of the U.S. Iodine deficiency is still a problem in other parts of the world and can cause mental retardation and other birth defects.

Sadly, areas of the world where soil iodine levels are low often also have low soil levels of selenium. This means that the combined deficiencies make low thyroid conditions even more prevalent in these areas. Furthermore, supplementing selenium alone without either an iodine supplement or thyroid hormone replacement worsens the hypothyroid condition, so some knowledge of thyroid status is needed when adding selenium to a food supplement program.

Another twist to the thyroid story is the observation that sometimes, in the conversion of T4 to T3 the wrong iodine atom is taken off. The result is production of an antagonist of T3 called reverse T3 (rT3). This form of the molecule shows up as T3 in the usual blood tests but is inactive. Therefore, if your physician is depending on a routine thyroid test panel to adjust your thyroid supplement, he may be led astray by the test results. It is possible, though expensive, to test for the presence of rT3 to see if this is complicating your situation.

In the decades before blood tests for thyroid status were available, early morning temperatures were used to check thyroid status. With the understanding that other factors besides thyroid status can affect temperature, this is still a viable option. If your waking temperature is under 97.4° and you don't know of any other explanation, you may have an underfunctioning thyroid and should consult your physician or an endocrinologist.

About every 10 years a new test for thyroid status is promoted as "the best" but to date there is no definitive single test. It may require a combination of tests and symptom evaluation to decide your true thyroid condition.

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