In hypothyroidism, the thyroid gland fails to produce adequate levels of thyroid hormone. Symptoms include the following:

Hashimoto’s thyroiditis is the most common natural cause of low thyroid hormone levels. In this autoimmune condition, the body develops antibodies that attack and gradually destroy the thyroid. A viral infection of the thyroid can also decrease thyroid hormone production, but the effect is generally mild and temporary. Finally, iodine deficiency can cause hypothyroidism, but this seldom occurs in the developed world where iodine is routinely added to salt.

Besides these natural causes, there is a still more common cause of hypothyroidism—medical treatment for hyper thyroidism (excessive production of thyroid hormone production). People with certain forms of hyperthyroidism receive treatment with radioactive iodine to inactivate the thyroid gland. This treatment causes hypothyroidism, which requires lifelong treatment with thyroid replacement therapy.

Until the 1990s, doctors commonly diagnosed hypothyroidism by conducting lab tests to measure thyroid hormone levels in the blood (the T ₄ level). Unfortunately, normal thyroid levels vary widely between people, so this method couldn’t always correctly identify the disease. A much better lab test, which became available in the 1990s, involves measurement of a hormone called TSH, or thyroid stimulating hormone.

TSH is released by the pituitary gland in order to control the thyroid gland. The pituitary gland constantly measures the level of thyroid hormone in the blood and adjusts TSH levels as necessary to get it right. When thyroid hormone levels are high, it turns TSH levels down. When thyroid hormone levels are too low, the pituitary raises TSH levels to stimulate the thyroid. If the thyroid gland does not respond by raising thyroid hormone levels, the pituitary turns up the TSH levels even higher. When TSH levels are higher than normal limits, this means that the thyroid gland is having trouble producing enough thyroid hormone for the body’s needs: in other words, the person has entered a hypothyroid state, or is about to enter such a state. This method of determining thyroid status has proved very reliable. In other words, the pituitary gland knows what it is talking about.

Medical treatment for low-thyroid conditions is safe and very effective. Treatment involves use of a hormone called levothyroxine, or T ₄ . The body actually uses two forms of thyroid, T ₄ and T ₃ , but in most cases the body easily and automatically converts T ₄ to T ₃ in the right proportions. The dosage of drug is adjusted by monitoring TSH levels. When the pituitary gland is satisfied, the dose is most likely correct.

So-called natural thyroid hormone is popular among people interested in alternative medicine. Sold by prescription under different brand names, this extract of pig (or mixed pork and beef) thyroid contains both T ₄ and T ₃ (see previous section for description). There is no doubt that extract is as effective as standard synthetic thyroid hormone, and it is a satisfactory choice for those who prefer to use natural treatments. However, there is no evidence that extract is any more effective than standard medications, and there are some concerns that variations in stomach absorption may produce slightly erratic results.

One double-blind study failed to find a combination of synthetic T ₃ and T ₄ more effective than synthetic T ₄ as a treatment for hypothyroidism in regard to well-being, quality of life, or mental function. 18 Another double-blind study failed to find any difference between T ₄ alone or T ₃ plus T ₄ in people whose thyroid had been removed because of thyroid cancer. 20 Yet another study also failed to find discernible differences between the two treatments regarding mood, fatigue, well-being, or mental function; however, for reasons that are unclear, patients given T ₃ plus T ₄ were significantly more likely to prefer the new treatment to their previous care than those who were continued on T ₄ . 19 Unless this was merely a statistical accident, people were apparently able to detect some subtle benefit with the combined treatment that they could not quite put their finger on.

Besides prescription extract, there are no natural therapies with documented efficacy for the treatment of hypothyroidism. Treatments that are sometimes recommended but lack any meaningful scientific support include Bacopa monniera (brahmi), traditional Chinese herbal medicine , vitamin B 3 , and zinc .

Selenium , though, may offer some benefits. 21 A review of 6 randomized trials, involving 339 people with hypothyroidism, compared selenium (200 mcg daily for 3 months) to placebo. Those in the selenium group reported improvements in their mood and well-being, and lab tests found lower levels of the antibodies that attack the thyroid.

Far too frequently, people with low thyroid levels consume seaweed or iodine supplements in hopes that they will help. However, while iodine deficiency does indeed cause low thyroid levels, taking iodine will not help at all if you are not deficient in it, and the vast majority of people living in the developed world have plenty of iodine. In fact, excessive iodine intake can occasionally cause hypothyroidism. This is a classic case of “more is not better.” (For more information, see Herbs and Supplements to Use Only With Caution below.)

As noted above, supplementation with iodine will not help the thyroid gland except in people who are iodine-deficient. In fact, in Japan, excessive use of seaweed (such as kelp or bladderwrack ) is a fairly common cause of hypothyroidism. 2-5

For this reason, people with low thyroid hormone levels should not consume excessive amounts of these iodine-rich foods.

Soy and its isoflavones (such as genistein ) appear to have numerous potential effects involving the thyroid gland. When given to people with impaired thyroid function, soy products have been observed to reduce absorption of thyroid medication. 6-8 In addition, some evidence hints that soy isoflavones may directly inhibit the function of the thyroid gland, although this inhibition may only be significant in people who are deficient in iodine. 9-11 However, to make matters more confusing, studies of healthy humans and animals given soy isoflavones or other soy products have generally found that soy either had no effect on thyroid hormone levels, or actually increased levels. 12-16

The bottom line: In view of soy’s complex effects regarding the thyroid, people with impaired thyroid function should not take large amounts of soy products except under the supervision of a physician.

Iron supplements may also interfere with thyroid hormone absorption. 17