The thyroid gland produces three hormones: thyroxine (T4), triiodothyronine (T3), and calcitonin. T4 and T3 are collectively referred to as “thyroid hormone” and regulate your metabolism, energy levels, and body temperature. Calcitonin plays a separate role, helping control calcium levels in your blood.
T4 and T3: The Main Thyroid Hormones
Thyroxine (T4) is the hormone your thyroid produces in the largest quantity, but it’s mostly a precursor. About 80% of the body’s active thyroid hormone, T3, actually comes from T4 being converted in other tissues like the liver and kidneys. Only about 20% of T3 is released directly from the thyroid gland itself. T3 is the more potent form, the one that enters your cells and switches on genes that control how fast your body burns energy.
The “4” and “3” refer to the number of iodine atoms attached to each hormone molecule. This is why iodine in your diet is essential for thyroid function. Adults need about 150 micrograms of iodine per day, which most people get from iodized salt, seafood, and dairy products. Without enough iodine, the thyroid simply cannot manufacture these hormones.
How These Hormones Affect Your Body
T3 works by binding to receptors inside the nucleus of your cells, where it activates genes that increase your metabolic rate. This translates to higher oxygen consumption, faster energy use, and more heat production. That chain of effects is why thyroid problems show up as such wide-ranging symptoms. Too little thyroid hormone and you feel cold, sluggish, and gain weight easily. Too much and your heart races, you lose weight without trying, and you feel overheated.
Beyond metabolism, thyroid hormones influence heart rate, digestion, brain development in children, bone turnover, and cholesterol levels. They touch nearly every organ system, which is why even small shifts in thyroid hormone levels can produce noticeable changes in how you feel.
Calcitonin: The Third Thyroid Hormone
Calcitonin is produced by specialized cells in the thyroid called C cells, which are distinct from the follicular cells that make T4 and T3. Its job is to lower blood calcium when levels rise too high, primarily by slowing the breakdown of bone. Calcitonin doesn’t affect your metabolism the way T4 and T3 do, and it isn’t included when doctors refer to “thyroid hormone” in general terms. It plays a relatively minor role in calcium balance compared to parathyroid hormone, which is produced by the separate parathyroid glands sitting behind the thyroid.
Calcitonin is more commonly relevant as a medical marker. Elevated calcitonin levels can signal medullary thyroid cancer, which originates in those C cells, so doctors sometimes order a calcitonin blood test when evaluating thyroid nodules.
Reverse T3: An Inactive Byproduct
Your body also produces a molecule called reverse T3 (rT3), which is a biologically inactive mirror image of T3. It’s created when T4 is converted in peripheral tissues, but instead of activating cells, reverse T3 may actually block T3 from binding to its receptors. During severe illness or extreme stress, reverse T3 levels tend to rise while active T3 drops, a pattern sometimes called “euthyroid sick syndrome.” Some practitioners argue that chronically elevated reverse T3 contributes to fatigue and other hypothyroid-like symptoms, though this remains debated in mainstream endocrinology.
How the Thyroid Makes Its Hormones
The thyroid is structured as millions of tiny spheres called follicles, each filled with a gel-like substance called colloid. This colloid is mostly thyroglobulin, a large protein that serves as the raw material and storage form for thyroid hormones. Iodine from your diet is absorbed in the small intestine, travels through the bloodstream to the thyroid, and gets concentrated inside these follicles.
An enzyme called thyroid peroxidase attaches iodine atoms to the thyroglobulin protein. When two iodinated segments of thyroglobulin couple together with a total of four iodine atoms, you get T4. When they couple with three iodine atoms, you get T3. The hormones sit stored in the colloid until the body needs them. When a signal arrives, the follicular cells pull thyroglobulin back inside, break it apart, and release the small T4 and T3 molecules into nearby capillaries.
How Your Body Controls Thyroid Output
Thyroid hormone production is tightly regulated through a feedback loop involving your brain. The hypothalamus, a region at the base of the brain, releases a signaling hormone called TRH, which tells the pituitary gland to release TSH (thyroid-stimulating hormone). TSH then travels to the thyroid and tells it to produce and release more T4 and T3.
When T4 levels in the blood rise high enough, the hypothalamus and pituitary detect this and dial back their signals, reducing TSH output. When thyroid hormone levels drop, TSH rises to push the thyroid to work harder. This is why a TSH blood test is the most common screening tool for thyroid disorders: a high TSH suggests your thyroid isn’t producing enough hormone, while a low TSH suggests it’s producing too much.
Standard lab reference ranges for TSH fall roughly between 0.4 and 4.0 mIU/L, with free T4 typically between 0.7 and 1.9 ng/dL. These ranges can vary slightly between laboratories and may shift with age and sex, which is why results are always interpreted alongside symptoms rather than in isolation.