Mother Earth Living

Herb Drug Interaction: Living with Hypothyroidism

By Robert Rountree, M.D.
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.


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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.

Soybeans (Glycine max) are another potential dietary influence on thyroid function. In one study of three infants born with hypothyroidism who were switched to a soy-based formula, researchers found that their dose requirement for levothyroxine increased significantly, only to drop again after they were taken off of the soy. Studies in adults have shown that eating soy protein at the same time as ingesting levothyroxine can impair absorption of the hormone; this effect goes away if the soy protein is consumed several hours after taking the hormone.

Many studies in adults have implicated soybean consumption as a cause of goiter (enlarged thyroid) in susceptible individuals, although it is not clear exactly how much soy a person would have to eat or how long they would have to eat it to develop this problem. One of these studies was conducted in Japan, where thirty-seven healthy people were given a daily dose of 30 g of whole soybeans for three months. At the end of this time, as many as half of them developed some symptom of low thyroid, such as goiter, malaise, constipation, or sleepiness. They also tended to have an increase in TSH levels to the high end of the normal range. The mechanism for this effect is not fully understood, although it is known that substances naturally found in soy can block the conversion of T4 to T3.

Millet, cassava root, pine nuts, and cruciferous vegetables such as broccoli, cabbage, kale, turnips, radishes, Brussels sprouts, and kohlrabi contain chemical compounds called goitrogens that keep the thyroid gland from using iodine to make T4. If eaten in large quantities for long periods of time, these foods can cause hypothyroidism and goiter. For example, in western Sudan, where millet is the staple food, there is a very high incidence of goiter. Cooking is supposed to inactivate most of these compounds, although it is difficult to find specific information on what temperature is necessary and how long the foods need to be heated for this effect to occur. Even less information is available on what happens to a person being treated for hypothyroidism when they consume these foods on a regular basis.

Herbs and thyroid function

Several medicinal herbs have also been shown to influence thyroid function. Ashwaganda (Withania somnifera), a traditional Ayurvedic tonic, has increased blood levels of thyroid hormones in laboratory mice in several studies. Guggul (Commiphora mukul), another Ayurvedic herb that has become a popular remedy for elevated cholesterol, has been found to increase T3. Presumably it does this by increasing the conversion of T4 to T3.

In contrast, lemon balm extract (Melissa officinalis) and rosemary (Rosmarinus officinalis) both contain a substance called rosmarinic acid, which has been found to bind to TSH and block its activity, thus having an inhibitory effect on the thyroid. Lemon balm has actually been used as a treatment for Graves’ disease, which is a type of hyperthyroidism. However, this activity is very weak and thus far has not been proven to cause thyroid problems in healthy individuals taking the herb for other reasons.

It is a safe assumption that research will reveal even more environmental and dietary influences on thyroid hormone metabolism. For example, laboratory studies suggest that flavonoids, which are ubiquitous in fruits and vegetables, can inhibit the conversion of T4 to T3. The amounts found in a typical diet may not be enough to cause concern, but what about the highly concentrated flavonoids, such as quercetin, that are available in supplement form? Is it possible that the use of these supplements is actually inducing hypothyroidism in the unsuspecting consumer?

A sensible approach

Given all of this complexity, what is a person with hypothyroidism (or the doctor who treats him or her) supposed to do? It would be easy to obsess about everything that could go wrong and get lost in the maze of choices. Ironically, the stress of worrying about which foods are safe to eat could itself worsen the hypothyroidism! Some experts take the easy way out and recommend avoiding ALL foods or supplements that have been reported to negatively—or positively—affect the thyroid. In my opinion, such approaches are unnecessarily extreme for most people.

The simplest advice is to always take thyroid hormone on an empty stomach, at least one or two hours before or one hour after any medications or mineral supplements, especially iron and calcium. Also, if a person has already been stabilized on thyroid replacement and makes any major dietary changes, such as going on a weight-loss program that involves daily intake of soy protein or lots of salads with raw, cruciferous vegetables in them, then it would be prudent to recheck blood levels of T4 and T3 on a regular basis. Hormone levels can definitely be useful as general guideposts for the treatment of hypothyroidism. However, in the final analysis, it is the way the person feels that counts the most.


Robert Rountree, M.D., is a physician in private practice in Boulder, Colorado, where he practices integrative medicine. He is co-author of Smart Medicine for a Healthier Child (Avery, 1994) and Immunotics (Putnam, 2000), and is an Herb Research Foundation advisory board member.


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