Your thyroid produces four hormones that control metabolism, heart rate, body temperature, and how your body uses energy

The thyroid gland makes two main hormones—T3 and T4—plus calcitonin and a small amount of reverse T3. T4 (thyroxine) is the storage form; your body converts it to T3 (triiodothyronine), which is the active form that actually does the work in your cells. Calcitonin regulates calcium in your blood. All four work together to keep your metabolism running at the right speed.

Understanding what each hormone does helps you make sense of thyroid blood tests, why symptoms happen when thyroid function changes, and what doctors are measuring when they check your TSH or free T4. The thyroid is small—about the size of a walnut—but these four hormones touch almost every system in your body.

Key Takeaways

  • T4 and T3 are the main thyroid hormones; T4 is inactive storage and T3 is the active form that your cells use.
  • Your body converts T4 to T3 in the liver, kidneys, and other tissues, so both hormones matter for how you feel.
  • Calcitonin regulates calcium levels in your blood and works with parathyroid hormones to keep bones and nerves working.
  • TSH (thyroid-stimulating hormone) is made by your pituitary gland, not your thyroid, but it controls how much T3 and T4 your thyroid releases.

T4 (Thyroxine): The Storage Hormone

T4 is the hormone your thyroid produces in the largest amount. It contains four iodine atoms, which is where the "4" comes from. T4 circulates in your blood attached to proteins that carry it around your body, and it is mostly inactive—it is the storage form. Your body does not use T4 directly; instead, your liver, kidneys, and other tissues strip off one iodine atom to convert T4 into T3, the active form.

When doctors test your thyroid, they often measure "free T4"—the small amount of T4 that is not attached to carrier proteins and is available for conversion. A normal free T4 level tells you your thyroid is producing enough raw material, but it does not tell you whether your body is converting it efficiently into T3. This is why some people feel hypothyroid symptoms even when their T4 is normal.

T3 (Triiodothyronine): The Active Hormone That Does the Work

T3 is the hormone your cells actually use. It has three iodine atoms instead of four, and it binds directly to receptors inside your cells to speed up metabolism, increase heart rate, and raise body temperature. T3 is much more potent than T4—it is about three to four times stronger. Your thyroid makes some T3 directly, but most of it comes from your body converting T4.

Free T3 is the form that matters for how you feel. If your T4 is normal but your free T3 is low, you may feel tired, cold, or sluggish because your cells are not getting enough active hormone. Some people convert T4 to T3 poorly due to stress, illness, nutrient deficiencies, or genetics, which is why two people with the same TSH can feel completely different.

Reverse T3: The Brake Hormone

Your body also produces reverse T3 (rT3), which is T4 with the iodine atoms removed in a different pattern than T3. Reverse T3 is inactive and actually blocks T3 from working—it acts like a brake on your metabolism. In normal amounts, reverse T3 helps fine-tune your metabolic rate. When you are stressed, very ill, or not eating enough, your body makes more reverse T3 to slow your metabolism down and conserve energy.

High reverse T3 can make you feel hypothyroid even when your T4 and T3 are normal, because the reverse T3 is occupying the receptors that T3 needs to attach to. This happens most often during prolonged stress, calorie restriction, or chronic illness. Some doctors measure reverse T3 to understand why a patient feels hypothyroid symptoms despite normal standard thyroid tests.

Calcitonin: The Calcium-Regulating Hormone

Calcitonin is produced by specialized cells in the thyroid called C cells. It works to lower calcium levels in your blood when calcium is too high. Calcitonin tells your bones to hold onto calcium and tells your kidneys to excrete more calcium in urine. It works alongside parathyroid hormone (made by the parathyroid glands, which sit behind your thyroid) to keep calcium in a narrow, safe range.

Calcium is critical for nerve signaling, muscle contraction, and bone strength. Most people do not think about calcitonin because it works quietly in the background, but when your thyroid is removed or damaged, calcitonin production drops and calcium regulation becomes harder. Calcitonin is also a tumor marker—very high levels can signal medullary thyroid cancer, which is why doctors sometimes measure it.

How TSH Controls Your Thyroid Hormones

TSH (thyroid-stimulating hormone) is not made by your thyroid—it is made by your pituitary gland at the base of your brain. TSH is the signal that tells your thyroid how much T3 and T4 to produce. When your blood T3 and T4 are low, your pituitary releases more TSH to wake up your thyroid. When T3 and T4 are high, your pituitary releases less TSH to slow production down. This is called a feedback loop.

TSH is usually the first test doctors order because it is sensitive and reflects the whole system at work. A high TSH usually means your thyroid is not producing enough hormone (hypothyroidism). A low TSH usually means your thyroid is producing too much (hyperthyroidism) or you are taking too much thyroid medication. TSH alone does not tell the whole story, which is why doctors also measure free T4 and sometimes free T3 or reverse T3.

Why Iodine Matters for All Four Hormones

All three active thyroid hormones—T4, T3, and calcitonin—require iodine to be made. T4 has four iodine atoms, T3 has three, and your thyroid cannot produce either without enough dietary iodine. Most people in the United States get enough iodine from iodized salt, dairy, and seafood, but iodine deficiency still occurs in some regions and in people who avoid salt or eat a very restricted diet.

Without iodine, your thyroid cannot make T4 or T3 no matter how hard it tries. Your pituitary will keep releasing TSH, your thyroid will keep trying, and you will end up with an enlarged thyroid (goiter) and hypothyroid symptoms. This is why iodine is added to table salt in many countries—it is a simple, cheap way to prevent thyroid disease at the population level.

Frequently Asked Questions

What is the difference between T3 and T4?

T4 is inactive storage; your body converts it to T3, which is active and does the actual work in your cells. T4 has four iodine atoms, T3 has three. Your thyroid makes both, but most T3 comes from conversion of T4 in your liver and kidneys.

Can you have normal TSH but still have a thyroid problem?

Yes. TSH reflects only the pituitary's signal, not how well your body is converting T4 to T3 or how much free T3 your cells are actually getting. Some people have normal TSH but low free T3 or high reverse T3 and feel hypothyroid symptoms.

What happens to these hormones if your thyroid is removed?

Your body stops making T4, T3, and calcitonin. You will need to take thyroid hormone replacement (usually synthetic T4) for life. Calcium regulation becomes harder without calcitonin, though your parathyroid glands can usually compensate.

Does stress affect how much T3 and T4 your thyroid makes?

Stress does not usually change how much T4 your thyroid produces, but it can change how your body converts T4 to T3. Chronic stress often increases reverse T3 production, which can make you feel hypothyroid even if your T4 is normal.

Why do some people need T3 medication in addition to T4?

Some people do not convert T4 to T3 efficiently due to genetics, illness, or nutrient deficiencies. Adding T3 medication (liothyronine) or using a combination medication (desiccated thyroid) can help if T4 alone does not relieve symptoms.