Thyroid hormones are responsible for stimulating the metabolic processes of every cell in the body. People can’t live without adequate amounts of thyroid hormones.

The thyroid gland, which sits in the lower part of the neck, produces about a teaspoon of hormones in an entire year. That means just a little variation in thyroid hormone production can have big effects on the body.

In order to grasp the effects of thyroid hormone, it is important to understand how the thyroid gland works. The gland is stimulated by a hormone from the pituitary gland — called, of course, thyroid stimulating hormone (TSH). The thyroid gland responds to TSH by producing a relatively inactive type of thyroid hormone called thyroxine or T4.

The enzyme deiodinase (also called iodide peroxidase)then converts thyroxine to the active type of thyroid hormone, triiodothyronine (T3), which performs thyroid hormone functions.

One condition that causes fatigue for many patients is a failure to convert inactive T4 into the active T3 hormones. I refer to these patients as “poor T4 converters.” There are many reasons why T4 fails or is slow to convert into T3, including hypoadrenalism. Adequate levels of cortisol and other adrenal hormones are necessary for the body to convert T4 to T3.

Other factors that can slow the conversion of T4 to T3 include insufficient levels of:

• B vitamins (especially vitamin B12)

• Iodine

• Iron

• Selenium

• Vitamin A

• Zinc

In addition, many commonly prescribed medications — such as beta-blockers, birth control pills, and antidepressants — can slow T4 conversion.

Fluoride, which is found in most public water sources, also is a potent inhibitor of thyroid activation. I have written many times about the dangers of fluoridating the water supply.

The most common reason a person would be unable to convert inactive T4 into active T3 is iodine deficiency — which affects that vast majority of our country. I estimate that more than 95 percent of Americans are iodine deficient.