The thyroid gland secretes hormones that regulate everything from your body temperature to your metabolism. In this series, we focus on three major, and common, thyroid issues—an underactive thyroid (hypothyroidism), an overactive thyroid (hyperthyroidism), and thyroid cancer. But first, it’s important to understand how thyroid hormone is produced and regulated, and how it exerts so many important effects on so many different organs. So enjoy this crash course in hormone physiology.
Your Thyroid Gland
The thyroid gland consists of two lobes joined in the center, resembling a bowtie, and sits low in the neck, just above the collarbones. You can easily palpate your thyroid gland in front of your trachea. The thyroid weighs 10 to 20 grams (for scale, a US nickel weighs about 5 grams). It is primarily made up of follicles, or small cavities, that are filled with colloid, a gel-like substance mostly composed of a protein called thyroglobulin. Men tend to have larger thyroids than women.
How Thyroid Hormone Is Made
The thyroid gland is the only organ in the body that utilizes iodine, which it gets from your diet. Foods high in iodine include seafood, dairy, kelp, seaweed, and iodized salt. When iodine attaches to the colloidal thyroglobulin, the iodinated parts undergo a chemical transformation that creates the two major thyroid hormones, T4 and T3 (the number refers to how many iodine molecules they contain). T4 and T3 are released into the bloodstream, where they circulate to all the organs and perform their magic.
Thyroid Hormones Make Cells Do Wonderful Things
Both T3 and T4 are active, but T3 is more potent and much of the T4 that is produced is converted to T3 in the peripheral tissues of the body. In the blood, T3 and T4 are attached to carrier proteins that protect them and serve as a reservoir for when the body needs them. The small portion of free, unattached thyroid hormone is the only hormone that is actually active.
Free thyroid hormone enters the cells of the body’s tissues and penetrates into the nucleus where it can switch on a variety of genes that control multiple aspects of bodily function. For example, thyroid hormone increases your heart’s ability to contract; the rate at which you breathe; the motility of your gastrointestinal system; the rate at which your bones remodel themselves; and even your mental alertness. In the fetus, thyroid hormone is essential for brain and skeletal development.
How Thyroid Hormone Levels Are Regulated
The body carefully controls how much thyroid is available to the various organ systems—too much or too little can be unhealthy and even dangerous. The pituitary gland, which sits at the base of your brain, senses the amount of thyroid hormone in the blood. It releases a hormone called thyroid-stimulating hormone (TSH), which turns on and off the production and release of T3 and T4 from the thyroid gland, depending on the body’s needs. Meanwhile, the pituitary gland is regulated by the release of thyroid-releasing hormone (TRH) from the neighboring hypothalamus. The levels of circulating thyroid hormone also affect TRH.
Finally, a lot of the more potent T3 is made outside the thyroid when it is converted from T4. The rate of this reaction is very responsive to the overall health of your body. For example, the conversion of T4 to T3 is enhanced by good nutrition, whereas it is diminished by starvation and diabetes.
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