Herb Drug Interaction: Living with Hypothyroidism


| March/April 2002



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Robert Rountree, M.D., is a physician in private practice and an Herb Research Foundation advisory board member.


As diseases go, hypothyroidism, or chronic thyroid deficiency, appears to be a cinch to diagnose and treat. That’s a good thing, because it is also a very common problem—more than 5 million Americans have been diagnosed with it. Most of them are treated with a synthetic version of thyroid hormone called Synthroid (levothyroxine). First introduced in 1959, Synthroid is the third most frequently prescribed drug in the United States. For the sixty years prior to Synthroid, the standard treatment was a desiccated (dried) concentrate of pig thyroid, which remains popular today, although not nearly as popular as its successor.

Because it controls metabolism and energy production, proper functioning of the thyroid gland is essential for health. When it isn’t functioning properly, metabolism slows way down, which can result in the “classic’’ symptoms of hypothyroidism, including fatigue, depression, forgetfulness, weight gain, fluid retention, constipation, dry skin, and hair loss. According to conventional medicine, the diagnosis of primary hypothyroidism must be confirmed by the presence of elevated blood levels of thyroid stimulating hormone (TSH).

TSH is made in the pituitary, a small gland near the center of the brain. When the thyroid gland doesn’t make enough T4, the pituitary increases its production of TSH in an attempt to make the gland work harder. In other words, thyroid hormone production is regulated by a simple negative feedback loop. When a diseased gland fails to make enough T4, the quick and easy solution is to replace it with a synthetic version.

A closer look

On the surface, this seems straightforward enough. However, a closer look reveals that there is a lot more to this picture. Recent reports in the medical literature have shown that absorption of synthetic T4 is strongly affected by dietary supplements. It also turns out that simply raising blood levels of T4 is often not enough to correct the symptoms of hypothyroidism. The reasons for this are complex but have to do with the fact that T4 must first be converted to yet another hormone called T3, which is a much more potent form.

This conversion process, which involves removal of a molecule of iodine, is influenced by a multitude of factors, including low-calorie diets, numerous prescription medications, chronic stress, certain toxins, and several botanical medicines. Deficiencies of zinc, copper, and selenium can also impair the conversion of T4 to T3. In other words, accurate diagnosis and optimal treatment of hypothyroidism isn’t such a simple task after all.

Dietary influences

Although it has been known for years that antacids, iron tablets, sucralfate (an anti-ulcer drug), and some laxatives could interfere with the absorption of levothyroxine, the impact of calcium supplements was not fully appreciated until recently. A study published in June 2000 looked at the effect of adding 1,200 mg of calcium carbonate tablets when people had already been on a stable dose of levothyroxine. The study showed that taking the calcium carbonate at the same time as the hormone resulted in a significant decrease in blood T4 levels and a significant increase in TSH, consistent with a worsening of thyroid function. Because concurrent treatment with both calcium and levothyroxine is quite common, especially in postmenopausal women, the implications of this study are huge, even though it is unknown how many people have actually developed symptoms as a result of the interaction.





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