Thyroid Function

• The thyroid gland
• The thyroid hormones
• The T4-to-T3 conversion process
• Thyroid Stimulating Hormone (TSH)
• Thyroid function links

The thyroid gland

The thyroid is part of the endocrine system, the system of organs that secrete hormones into the blood. A butterfly-shaped gland wrapped around the front of the larynx, it is the largest endocrine gland and the first to appear in the fetus during pregnancy. Its function is to synthesize the thyroid hormones T4 and T3 from iodine.

Follicles (round sacs) in the thyroid are filled with thyroglobulin, a thyroid protein. When dietary iodine enters the thyroid, it travels to the follicles, where T4 and T3 are made. These thyroid hormones are first stored in the thyroglobulin and then released into the bloodstream. A complex system of proteins and other molecules carries the thyroid hormones to the cells.

When the thyroid is underproducing, the action of the TSH hormone stimulates it to produce more. This action may enlarge the thyroid when not enough iodine is present. A swollen or enlarged thyroid is called a goitre (also spelled "goiter"). The prevalence of goitres is higher in areas of iodine deficiency; hyperthyroidism and some other causes can also result in goitres.

Nodules are lumps that sometimes grow on the thyroid. About 95 percent of them are benign. Because of the small risk that they may be cancerous, they should be checked by a doctor.

The thyroid hormones

Most prescriptions for hypothyroidism replace only one thyroid hormone — thyroxine, or T4. Yet the thyroid gland produces four hormones. How many do we need?

The two main thyroid hormones are thyroxine (T4, containing four iodine atoms) and triiodothyronine (T3, containing three iodine atoms). T4 is a slow-acting pre-hormone. Ingested T4 takes four days to peak in the blood, and the overall effects aren't reached for about six weeks. This slow, steady action is part of why most doctors prefer to prescribe it alone instead of with T3.

T4 is stored in the thyroid gland and in the bloodstream. It's a reservoir of hormone available to be converted to T3, the active and faster-acting hormone. The immediate effects of T3 last for one to two days.

The thyroid hormones travel through the bloodstream to every cell in the body, where they keep everything working optimally, like oil in an engine. Without enough thyroid hormone in its active state (T3), the metabolism slows down, resulting in lowered pulse rate, coldness, weight gain, and fatigue. Other body functions slow down as well — the mind can't function normally, skin becomes dry and flaky, hair falls out, muscles become cramped, and so on.

Additional thyroid hormones are T2 and T1. In medication form, they may be found only in desiccated thyroid, which is natural thyroid hormone derived from pigs' thyroids. Much less is known about these hormones.

Many thyroid patients do quite well on T4 medication alone, or on T4 and T3 together. However, for those that don't, desiccated thyroid is a viable option. The majority of patients who switch to desiccated thyroid find that their health improves, so these unmeasured hormones appear to have some function.

The T4-to-T3 conversion process

About 20 percent of the body's T3 comes from the synthesis of T3 within the thyroid. The rest of the T3 is derived when an enzyme (called a deiodinase) removes one iodine molecule from T4. While T4 is converted to T3 in a number of locations in the body, the main place that this occurs is in the liver.

All the thyroid hormone metabolic activity comes from T3. Except during pregnancy, when T4 is needed for the fetus to develop properly, T4 is in the body just to be converted to T3. In a normally-functioning thyroid, enough of this conversion process takes place. Most people with hypothyroidism do not have normally-functioning thyroids, however.

Imagine you go to a foreign country with a stack of money in your country's currency to convert into the local currency when you get there — and then, for some reason, you're unable to convert it. You're loaded with cash, but you can't spend it. The situation is similar for those patients on T4-only medication who may not be able to able to convert that T4 adequately into T3.

This isn't a problem for everyone with hypothyroidism, but lab test results sometimes don't indicate a conversion problem when there is one. The majority of those on T4 alone who still have symptoms of hypothyroidism benefit from taking T3 in a natural or synthetic form, usually in addition to T4.

Thyroid Stimulating Hormone (TSH)

The pituitary is an endocrine gland located at the base of the brain. It is exquisitely sensitive to the thyroid hormone levels in the blood. TSH (thyroid stimulating hormone), a pituitary hormone, stimulates all aspects of thyroid function from synthesizing thyroid hormones to releasing them into the bloodstream. When the thyroid hormone level drops, the pituitary produces and releases more TSH. The thyroid releases more T4 and T3 into the bloodstream in response. As the thyroid hormone levels rise, the pituitary reduces or shuts off TSH. This feedback mechanism keeps the level of thyroid hormones in a fairly constant range with a normally-functioning thyroid.

A high TSH level should indicate low thyroid hormone levels, and a low TSH level should indicate optimum or high thyroid hormone levels. However, although TSH is part of the endocrine system, it is not a thyroid hormone. Precisely what TSH level indicates in each body that it isn't getting enough thyroid hormone? What if the thyroid is producing enough T4, but the body isn't converting enough of that T4 to T3? Why assume that the pituitary and the hypothalamus, which controls the pituitary, are functioning optimally when another part of the endocrine system isn't? If they're malfunctioning, the feedback mechanism can't work.

The medical establishment decided in about 1974 that the TSH test was the test to diagnose hypothyroidism and hyperthyroidism, and to determine the amount of medication needed. This was apparently done without any testing to see if TSH lab test results correlated with symptoms. In fact, diagnosing according to symptoms was put aside in favour of getting a diagnosis from a lab.

In addition to the well-documented problems with the lab ranges with this test, the TSH level is simply not a reliable indicator of thyroid function for many people.

Thyroid function links

At this site

TSH References
Pages of references showing the problems with the lab ranges used for the TSH test.

T3 References
Pages of references showing the clinical effectiveness of using T3 in addition to T4 to treat hypothyroidism for many patients.

Desiccated Thyroid References
Pages of references showing that for many thyroid patients, desiccated thyroid is more effective in relieving hypothyroidism symptoms than synthetic T4 is..

At other sites

How Your Thyroid Works at endocrineweb.com

Thyroid Dysfunction at the Mary Medical Clinic website
Discusses diagnosis difficulties, T4-to-T3 conversion problems, and Armour.

Thyroid Disease Manager
Written for doctors, this site "...offers an up-to-date analysis of thyrotoxicosis, hypothyroidism, thyroid nodules and cancer, thyroiditis, and all aspects of human thyroid disease and thyroid physiology."