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by Jeffrey S. Bland, Ph.D
Iodide isn't enough to keep your thyroid healthy. Proper intake of selenium, iron and essential fatty acids hold the key to a healthy thyroid and metabolism.
When manufacturers began to fortify salt with iodide to prevent goiters, many people breathed a sigh of relief and assumed all thyroid disorders were resolved. Evidence over the past 10 years, however, demonstrates this assumption is far from justified. Health problems as diverse as learning disabilities in children and heart disease in older people that may be related to disorders of the thyroid gland still crop up. Dietary deficiencies and a buildup of toxins in the body can contribute to the problem.
Residing at the base of your throat in the center of your chest, your thyroid gland controls the speed of your metabolism. It receives signals from the hypothalamus and pituitary glands in the brain. These glands send out chemical messengers or releasing factors that signal the thyroid gland to speed or slow its activity. The thyroid gland in turn secretes the hormone thyroxin, which then tells the body's tissues how fast to break down food to produce energy. The fact that thyroxin contains four atoms of iodine explains the need for adequate dietary iodide to support proper thyroid function.
The most interesting recent discovery is that the hormone thyroxin isn't the most active regulator of metabolic function derived from the thyroid gland. Instead, that distinction belongs to thyroxin's close relative, the hormone triiodothyronine, or T3. T3 is produced from thyroxin by tissues such as the liver, muscles and heart. In essence, it's as though the thyroid gland "talks" to the tissues of the body through their conversion of the thyroid hormone thyroxin to T3, and T3 helps regulate the speed of metabolism in the tissues (New England Journal of Medicine, 1979, vol. 300).
A deficiency of iodide in the diet can therefore prevent the thyroid gland from producing enough thyroxin, which in turn inhibits the tissues and organs of the body from making enough T3. In an extreme case of deficiency, the thyroid gland swells, all organs of the body are adversely affected, and the result can be enlargement of the heart and subsequent death.
In more chronic cases, long-term insufficiency of iodide in the diets of children can lead to learning disabilities and poor motivation (American Journal of Clinical Nutrition, 1996, vol. 63). Similarly, iodide deficiency in the diet of a pregnant woman can compromise her child's brain development, which indicates iodide's important role in influencing human thyroid function and brain development.
The effects of diet and environment on thyroid function are more complicated than iodide alone, however. In recent research, scientists determined that the conversion of thyroxin to T3 is also controlled by the essential trace mineral selenium (American Journal of Clinical Nutrition, 1993, vol. 57). For some time, scientists have known selenium is critical for human health because it plays a role in the body's antioxidant system, regulating the enzyme glutathione peroxidase. The discovery that selenium also activates an enzyme responsible for controlling thyroid function by the conversion of thyroxin to T3 was considered a breakthrough, since selenium deficiency might be a symptom of thyroid problems (Biology and Trace Element Research, 1992, vol. 33).
Even more recently, low selenium status in elderly people has been associated with reduced thyroid function and fatigue, skin problems, depression and reduced immunity (Clinical Science, 1995, vol. 89).
Exercise caution in evaluating your need for iodide and selenium because humans require only a few micrograms of these nutrients each day to meet normal needs. (A microgram is one-millionth of a gram, which is less than the weight of the ink used in the word "microgram.") Too much of either of these nutrients can produce toxicity, symptoms of which may be similar to those caused by insufficiency of these minerals. Nutritional tests such as analysis of blood, serum or hair are useful in determining an individual's iodide and selenium status if he or she has signs of insufficiency.
Other nutritional factors also contribute to thyroid function including overactivity of the thyroid gland (hyperthyroidism) or thyroid underactivity (hypothyroidism). One of these nutritional factors is the production of toxic substances in the intestinal tract from certain forms of bacteria. These toxic substances travel to the liver from the intestines and impair the liver's ability to manufacture and utilize T3, resulting in some of the signs of low thyroid function (American Journal of Physiology, 1995, vol. 268).
Pollution and Thyroid Disorders
Toxins from the environment also alter our thyroid function. Eduardo Gaitan, M.D., from the University of Mississippi School of Medicine, found that drinking water contaminated with organic hydrocarbons results in blocked activity of thyroid hormones and can produce symptoms of hypothyroidism (Journal of Clinical Endocrinology and Metabolism, 1983, vol. 56).
Polychlorinated biphenyls (PCBs) can alter thyroxin levels and result in symptoms of thyroid disorders (Science, 1995, vol. 267). These pollutants block the metabolism of thyroid hormones and may contribute to toxicity through the development of chemically induced porphyria, a condition that produces symptoms such as the mental illness that afflicted King George III (Human Experimental Toxicology, 1994, vol.13).
We now understand that the environment, diet and nutrition influence thyroid function in a number of ways and may relate to thyroid disorders of nonspecific origin (Annual Review of Nutrition, 1995, vol. 15). Poor iron status, which results in anemia and poor tolerance to cold, for example, results in lowered thyroid function (American Journal of Clinical Nutrition, 1990, vol. 52). Fasting and anorexia nervosa, both of which are associated with nutrient depletion and deficiencies, also decrease thyroid function (American Journal of Clinical Nutrition, 1985, vol. 42).
In situations in which thyroid function is reduced, the body tends to accumulate carotenes in the skin. (Carotenes are the pigments from orange-red fruits and vegetables). If a person's skin suddenly takes on an orange coloring without provocation and not as a result of liver disease, thyroid function should be investigated.
Watch the Foods You Eat
Eczema or other skin problems are also symptoms of poor thyroid function. The skin's integrity depends on metabolism of essential fats in the diet. Low thyroid function results in poor utilization of these fats in the maintenance of skin integrity (Journal of Nutrition, 1995, vol. 125).
Some types of foods are reputed to be goiterogenic, which means they create problems with thyroid hormones. Common foods with this reputation are members of the brassica family (which includes cauliflower, Brussels sprouts, cabbage and broccoli) and soyfoods. The brassica family causes problems with the metabolism of thyroid hormones only when they're consumed in high amounts such as when a person regularly drinks cabbage juice (Natural Toxins, 1995, vol. 3).
With soy products, the story isn't as clear. Some infants who consume soy formula do develop thyroid problems (Pediatrics, 1995, vol. 96). In addition, thyroid problems are more prevalent among infants who consumed soy formula than in those who consumed breast milk (Journal of the American College of Nutrition, 1990, vol. 9). What's not clear is the mechanism underlying these changes in thyroid function following consumption of soy-based formula. In part, this may be due to a soy allergy that cross-reacts with the thyroid gland or to unique phytochemicals in soy that influence thyroid hormone metabolism (Journal of Nutrition, 1995, vol. 125). When adults eat soy products, their thyroxin levels rise, so there may be substances in soy which directly influence thyroid activity.
The recognition that diet and environment play important roles in determining thyroid function opens up the possibility of new approaches to improving thyroid function in people for whom the thyroid is either too active or not active enough. If you're concerned about your thyroid function, see your doctor for thyroid screening, which consists of a simple blood test.
Symptoms of Thyroid Dysfunction