What is the Thyroid and How Does It Work?

by Mary Shomon Patient Advocate

The thyroid is known as the “master gland of metabolism.” Healthy thyroid function is crucial to every bodily function, and every organ, gland, tissue, and cell in your body. Let’s take a look at the thyroid, its function, and how it works.

Labeled thyroid drawing

What is the thyroid?

Your thyroid — along with other glands like the ovaries, testes, pancreas, and adrenals, for example — is part of your endocrine system. Like all glands, the thyroid manufactures and releases hormones that perform key functions.

Your thyroid is shaped like a butterfly, and the two “wings” — known as lobes — are connected by a narrow strip of skin called the isthmus.

A normal adult thyroid gland weighs around one ounce. The thyroid is typically larger in women and enlarges during pregnancy.

Thyroid image

Where is your thyroid located?

Your thyroid is located in the front of your neck, just below your Adam’s apple (larynx) area.

Each of your thyroid’s lobes is located on either side of your trachea (windpipe). Each lobe has multiple small lobules, each containing multiple follicles. These follicles store thyroid hormones.

Your thyroid is covered by fibrous tissue called fascia that makes it move when you swallow.

Thyroid hormones image

What does your thyroid gland do?

Your thyroid gland’s key function is to produce hormones. The two key hormones it produces are:

  • Thyroxine (T4), which has four atoms of iodine
  • Triiodothyronine (T3), which has three atoms of iodine

The purpose of these thyroid hormones is to help facilitate metabolism — the conversion of oxygen and calories to energy — and the delivery of that energy to all the cells in your body.

The thyroid gland also produces calcitonin, a hormone that helps to control the body’s levels of calcium.

whole body effect

What do thyroid hormones do?

Specifically, T3 and T4’s role is to regulate the speed of your metabolism and bodily functions by transporting energy in the form of glucose to your cells.

Thyroid hormone, for example, supports, maintains, and promotes:

  • Your body temperature
  • Your heart and brain function
  • The absorption and digestion of food
  • The growth of your bones, hair, skin, and nails
  • Your normal reproductive function
  • Your nervous system and reflexes
  • Every organ, gland, tissue, and cell function in your body
Salting food for iodine

How does the thyroid make thyroid hormone?

Your thyroid gland takes iodine from food and supplements, adds the amino acid tyrosine (from your diet) and — with the help of an enzyme called thyroperoxidase (TPO) that is made in the thyroid gland — produces and releases thyroid hormones.

The normal thyroid gland produces about 80 percent T4 and about 20 percent T3. T4 is a pro-hormone and needs to be converted into the active T3 hormone in your thyroid and other tissues so it can be used by cells.

Erythrocyte in the vein

What happens when thyroid hormones are released?

After they are released into your bloodstream, most of your thyroid hormone is attached to proteins and isn't available to your cells. The available — or “free” — thyroid hormones travel through your bloodstream, cross cell membranes, and facilitate the delivery of energy to your cells.

The release of thyroid hormones is part of a complex feedback loop that involves your pituitary gland and your hypothalamus.

Labeled pituitary gland

Your pituitary’s role

Think of your pituitary — a small, peanut-sized gland in your brain — like a thermostat. A thermostat maintains a specific temperature by turning the heat on or off. Similarly, your pituitary maintains thyroid hormone levels by turning thyroid hormone production on and off. When the pituitary detects low levels of thyroid hormone, it releases thyroid stimulating hormone (TSH), telling the thyroid to make more hormone. When the pituitary detects too much thyroid hormone, it decreases TSH production.

Image of Brain labeled

The role of your hypothalamus

Your pituitary gland is controlled by yet another gland in your brain called the hypothalamus. The hypothalamus produces TSH releasing hormone (TRH). TRH tells your pituitary gland to stimulate the thyroid gland and release TSH. Again, like a thermostat, the hypothalamus produces more or less TRH in response to levels of thyroid hormone, to help you maintain a healthy balance of thyroid hormones.

When you don’t have enough thyroid hormone

There are a number of reasons why your thyroid may not be able to produce enough thyroid hormone, a condition known as hypothyroidism. When you do not have enough thyroid hormone, your organs, glands, tissues, and cells become starved for energy and processes slow down. Symptoms of hypothyroidism include fatigue, slowed thinking, low heart rate and blood pressure, slowed digestion, and weight gain, among others.

When you have too much thyroid hormone

There are a number of reasons why your thyroid may produce too much thyroid hormone, a condition known as hyperthyroidism. When you have too much thyroid hormone, your organs, glands, tissues, and cells become overstimulated with excess energy and processes speed up unnaturally. Symptoms of hyperthyroidism include insomnia, anxiety, rapid heart rate, high blood pressure, diarrhea, and weight loss, among others.

Mary Shomon
Meet Our Writer
Mary Shomon

Mary Shomon is a patient advocate and New York Times bestselling author who empowers readers with information on thyroid and autoimmune disease, diabetes, weight loss and hormonal health from an integrative perspective. Mary has been a leading force advocating for more effective, patient-centered hormonal healthcare. Mary also co-stars in PBS’ Healthy Hormones TV series. Mary also serves on HealthCentral’s Health Advocates Advisory Board.