Thyroid disease is one of the most common medical conditions in the United States today. It is estimated that 27 million – 1 in 10 – Americans has some degree of thyroid dysfunction. It is said that nearly half of these individuals are not aware that they have thyroid disease. This is largely because few physicians are willing to look beyond two basic blood tests to determine whether a thyroid problem exists.
Regulation of thyroid hormone production begins in the area of the brain called the hypothalamus, which produces a chemical called thyrotropin releasing factor. TRF stimulates the production of a second hormone, thyroid stimulating hormone (TSH) by the pituitary gland. TSH in turn triggers the manufacture of multiple hormones by the thyroid gland.
The primary hormone produced by the thyroid gland is called levothyroxine, commonly referred to as T4. The thyroid produces other forms of thyroid hormone including triiodothyronine (T3), diiodothyronine (T2), and monoiodothyronine (T1), but in smaller quantities. As the production of thyroid hormone increases, the release of TSH from the pituitary gland decreases. Thus proper balance of thyroid hormone production is maintained.
The interrelationship of the various thyroid hormones in the body is quite complex. T4 is primarily a storage form of the hormone upon which the body can draw to produce the other forms of thyroid hormone as needed. It is converted to T3, the primary active form of thyroid hormone, at the tissue level. T3 is critical to the ability of cell nuclei to direct the activities within cells. It is also needed for cellular energy production.
The conversion of T4 to T3 is accomplished through the action of two enzymes known as D1 and D2. When D1 and D2 activity is low, conversion of T4 to T3 is insufficient to meet the body’s needs and signs and symptoms of hypothyroidism appear. In recent years it has been discovered that D2 activity is enhanced by the presence of T2, a form of thyroid hormone that has traditionally been considered inactive and of no importance.
It has also been found that T2 plays a crucial role in energy production in the liver, heart, and skeletal muscles. T2 acts quickly and directly to increase the metabolic rate by facilitating oxygen use, while T3 acts more slowly and indirectly through the activation of various enzymes. While very little is known about T1 activity, it now appears likely that it too is an active hormone, and not simply a breakdown product of T4 and T3.
In an individual with normal thyroid function a sequence of events is always occurring. Under the influence of TSH the thyroid gland is producing T4, the storage form of thyroid hormone, T3, the primary active form of thyroid hormone, T2, the form of thyroid hormone required to facilitate the conversion of T4 to T3 at the cellular level, and T1, of as yet unknown significance. As the levels of these hormones rise, pituitary production of TSH falls. When the production of thyroid hormone drops below an optimum level the pituitary gland increases the release of TSH. This feedback mechanism keeps thyroid activity in balance.
When the thyroid gland is unable to produce optimum amounts of thyroid hormones the TSH level will remain high. An elevated TSH level is the criterion that must be met before most physicians will entertain a diagnosis of hypothyroidism. If an individual is unable to effectively convert stored T4 to the active hormone T3 or if thyroid antibodies that block thyroid hormone activity are present the TSH level will not rise. That person will exhibit the signs and symptoms of hypothyroidism, but the diagnosis will not be made and treatment will not be instituted.
It is not necessary that all forms of thyroid hormone be present for pituitary release of TSH to return to normal levels. Any form of thyroid hormone is independently capable of telling the pituitary gland that thyroid activity is adequate. Therefore administration of T4 alone, T3 alone, or even T2 alone will effectively normalize TSH levels.
This presents a serious challenge for individuals who have lost normal thyroid function. If levothyroxine (T4), the storage form of thyroid hormone, is supplemented the TSH level will fall. It is therefore possible to normalize levels of both T4 and TSH with supplementation of levothyroxine alone. In the absence of T2, however, T4 will not be efficiently converted to T3 and thyroid hormone activity will remain low. This is why many people who take levothyroxine alone continue to experience signs and symptoms of hypothyroidism while exhibiting perfectly normal and balanced TSH and T4 blood test results.
Very few physicians are aware of the role of T2 in the body. Some recognize that T4 conversion to T3 must take place for thyroid function to be restored, but most do not. They prescribe levothyroxine, check T4 and TSH blood levels, and assume that anyone who continues to complain of feeling tired, cold, constipated, and losing hair is suffering from depression or some other malady totally unrelated to thyroid activity.
Desiccated thyroid, which is extracted from animal thyroid glands, contains all forms of thyroid hormone produced by a normally functioning thyroid gland. Since T2 is available, the body can effectively convert the T4 that is present to T3 and normal thyroid function is restored.