Hyperthyroid Disorders

Lesson 1: What is Hyperthyroidism?

Thyroid Hormones

The two major hormones produced by the thyroid gland include: thyroxine, which is commonly known as T4; and triiodothyronine, which is known as T3.

Thyroid hormone is composed of the amino acid tyrosine and the mineral iodine. T4 contains one atom of tyrosine attached to four iodine atoms. T3 contains one atom of tyrosine and three iodine atoms.

The thyroid gland produces primarily T4 and a smaller amount of T3 along with the precursor thyroid hormones T1 and T2. Although their concentrations are not routinely measured, T1 and T2 also have physiological functions. While T1 has not been well studied, T2 is needed to produce several enzymes needed for proper thyroid gland functioning.

Although the thyroid gland produces T3, most of the body’s supply of T3 is converted from T4 in the body’s organs by a process called de-iodination. In this process T4 molecules lose one iodine atom to form T3.

The thyroid hormone T3 is reported to be approximately 10 times more potent than T4. It’s thought that T4 has little biological activity, and that its primary role is to act as a pro-hormone and supply freshly converted T3. It’s known that the conversion of T4 into fresh T3 occurs in several target organs including the brain and liver. The brain and pituitary gland require freshly converted T3 for their optimal function.

Thyroid hormone travels through the body linked to protein molecules. Linked to protein, thyroid hormone is known as T4 and T3. T4 and T3 do not have any biological activity. T4 and T3 must be first split apart or cleaved free from the protein molecules. In its free form, thyroid hormone is known as free T4 or FT4 and free T3 or FT3. FT4 and FT3 have biological activities and they're able to react with the body's cells.

The T4 and T3 tests, which measure free hormone along with the protein molecules that it's linked to, may be falsely high or low if the body’s stores of binding protein are high or low. Binding protein levels are high in pregnancy and in the presence of many medications including estrogens.

Free Thyroid Hormone: Only free T4 and free T3 are active. That is, they’re able to cause the actions associated with thyroid hormone. Blood tests for free T4 are called FT4 tests, and blood tests for free T3 are called FT3 tests. Now that direct measurements of FT4 and FT3 are available, these tests are considered superior to assays for T4 and T3.

Thyroid hormones initiate a chain of molecular events or gene expressions when the active form of the hormone interacts with specific cell receptors and subcellular components of various organs.

These receptors have been found in the liver, brain, heart, spleen, testis, kidney and anterior pituitary gland with the liver having the largest number of receptors and the pituitary the least. In summary, active or free thyroid hormone molecules react with specific cell components or binding sites on specific organs. Here, they initiate specific cellular changes, such as energy production, that are also influenced by genes and other factors.

To complicate matters, the receptor sites for thyroid hormone are shared with other substances, such as other hormones, including the hormone vitamin D. This is why someone with hyperthyroidism who has the same thyroid hormone levels as another individual will not have identical symptoms.

Thyroid hormones stimulate many metabolic processes, including energy expenditure and the basal metabolic rate, including digestion, and the metabolism of lipids, carbohydrates, drugs, proteins and minerals. Thyroid hormone is necessary for the proper functioning of other glands and organs. People deficient in thyroid hormone typically have low levels of other hormones.

When levels of either FT4 or FT3 are low, symptoms of hypothyroidism occur. When levels of either FT4 or FT3 are high, symptoms of hyperthyroidism occur. For diagnosing these disorders, blood tests for FT4 and FT3 are superior to tests for T4 and T3 since the latter two tests are influenced by binding protein levels.

When FT3 is elevated and FT4 is normal, this form of hyperthyroidism is called T3 thyrotoxicosis. T3 thyrotoxicosis is a common occurrence in hyperthyroidism caused by Graves’ disease and also toxic multinodular goiter.

In the next section you’ll learn about one of the body’s major axis systems and how it regulates thyroid hormone levels.

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