The thyroid is one of the largest endocrine glands in the body and specifically controls how quickly the body uses energy, how it makes proteins, and the body’s sensitivity to other hormones. The function of the thyroid gland is to take iodine and convert it into thyroid hormones — primarily, thyroxine (T4) and triiodothyronine (T3). Normal thyroid cells accumulate and retain iodide far more efficiently than do any other cells in the body. Most cells don’t absorb iodine at all, but some, including thyroid cancer cells and breast epithelial cells, can to a limited degree. Thyroid cells combine iodine and the amino acid tyrosine (as bound to thyroglobulin) to make T3 and T4. T3 and T4 are then released into the bloodstream and transported throughout the body, where they control metabolism (i.e., the conversion of oxygen and calories to energy). Every cell in the body depends upon thyroid hormones for regulation of their metabolism.
Anatomically speaking, the thyroid is a butterfly shaped gland (two larger lobes connected by a narrower isthmus) located between the Adam’s apple and the clavicle. A normal thyroid gland cannot be felt externally. If a doctor can “see” it or “feel” it when touching the neck with his fingers, it’s enlarged. Under normal circumstances, it’s soft and flat.
When talking about thyroid hormones, we’re actually talking about four bio-chemicals:
• Thyroglobulin is a protein (not a hormone) produced by the thyroid. It is synthesized from amino acids and iodide and stored in the follicular lumen as a colloid and used entirely within the thyroid gland in the production of the thyroid hormones.
• T3 (triiodothyronine) affects almost every physiological process in the body, including growth and development, metabolism, body temperature, and heart rate. Production of T3 and its prohormone, T4 (T3 is actually produced by the breakdown/conversion of T4), is activated by thyroid-stimulating hormone (TSH), which is released from the pituitary gland. As a side note, the 3 in its name refers to the fact that it contains 3 iodine atoms.
• T4 (thyroxine, AKA tetraiodothyronine) is the prohormone from which the body creates T3. It is synthesized from residues of the amino acid tyrosine, found in thyroglobulin. Every cell in the body depends upon the thyroid hormones T3 and T4 for regulation of their metabolism. The normal thyroid gland produces about 80% T4 and about 20% T3. However, T3 is about four times “stronger” than T4. T4 is converted to T3 in body cells. This allows the body to fine tune the metabolic regulating capabilities of T3 and T4. As with T3, the 4 in T4’s name refers to the fact that T4 contains 4 iodine atoms.
• Calcitonin is produced in the parafollicular cells and regulates calcium levels in the blood (to a minor degree), along with the parathyroid glands (the main regulator). It lowers blood calcium and phosphorus by decreasing the rate of re-absorption of these minerals from bone.
Thyroid chemistry is an iodine-based chemistry; iodine must be ingested because it can’t be manufactured in the body; it is an element, not a compound. In fact, follicular cells actively trap virtually all iodine/iodide molecules in the body. Any iodine you ingest is trapped exclusively by cells in the thyroid to be used for manufacturing thyroglobulin and, ultimately, T3 and T4. This fact is exploited by endocrinologists when it comes to treating several thyroid disorders. If iodine is not present in sufficient amounts, the body will develop a benign goiter (enlargement of the thyroid) over time. It is common in areas where iodine does not naturally occur in food. In the early 1900’s, Western countries began adding iodine to salt to combat this problem. And it worked, in the sense that goiters are now uncommon in the Western world.
Thyroid-stimulating hormone (TSH) from the anterior pituitary regulates the processes via a negative feedback loop. That is to say, thyroid releasing hormone (TRH) from the hypothalamus stimulates the pituitary to release TSH into the bloodstream, which stimulates thyroid follicular cells to add iodine to the amino-acid (tyrosine) component of thyroglobulin (which, once again, is stored as colloid within the lumen of the thyroid follicles).
Once converted, the T3 and T4 hormones are released into the bloodstream. This arrangement essentially works as a reserve system for thyroid hormones, allowing it to release active hormones into the body on an as needed basis. As more thyroid hormones are produced, blood levels of T3 and T4 rise. Ultimately, these hormones make their way through the bloodstream back to the hypothalamus, telling the hypothalamus that enough is enough and to stop releasing TRH, which stops the pituitary from releasing TSH — shutting down the cycle.
Thyroid hormones regulate the following activities:
• Oxygen uptake (they upregulate it).
• Gross basal metabolic rate (they upregulate it).
• Maintenance of body temperature.
• Intracellular metabolism (microscopic protein synthesis, lipid breakdown, and cholesterol breakdown.) Patients who are hypothyroid, for example, will have higher levels of cholesterol in the blood because of reduced thyroid up regulation. Patients who are hyperthyroid will often be thin and have lower levels because of too much up regulation.
• Growth and development; that is, body growth rate and nervous system development.
• Thyroid hormones also enhance the effects of catecholamines, accounting for high blood pressure, nervousness, sweating, and fast heart rate in hyperthyroid patients.