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Everything You Need to Know About Your Thyroid

Nov 20, 2014
By Malcolm Thaler

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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Malcolm Thaler

Malcolm enjoys being on the front lines of patient care, managing diagnostic and therapeutic challenges with a compassionate, integrative approach that stresses close doctor-patient collaboration. He is the author and chief editor of several best-selling medical textbooks and online resources, and has extensive expertise in managing a wide range of issues including the prevention and treatment of cardiovascular disease, diabetes, and sports injuries. Malcolm graduated magna cum laude from Amherst College, received his MD from Duke University, and completed his residency in Internal Medicine at Harvard's New England Deaconess Hospital and Temple University Hospital. He joined One Medical from his national award-winning Internal Medicine practice in Pennsylvania and was an attending physician at The Bryn Mawr Hospital since 1986. He is certified through the American Board of Internal Medicine. Malcolm is a One Medical Group provider and sees patients in our New York offices.

The One Medical blog is published by One Medical, a national, modern primary care practice pairing 24/7 virtual care services with inviting and convenient in-person care at over 100 locations across the U.S. One Medical is on a mission to transform health care for all through a human-centered, technology-powered approach to caring for people at every stage of life.

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