The thyroid produces two main hormones: T4 and T3

Your thyroid gland makes two hormones that control how fast your body uses energy. The first is called T4 (thyroxine), which is the hormone your thyroid releases in the largest amount. The second is T3 (triiodothyronine), which is more powerful but produced in smaller quantities. Both hormones travel through your bloodstream to nearly every cell in your body, telling those cells how quickly to work.

The thyroid also produces a third hormone called calcitonin, which helps regulate calcium levels in your blood and bones. This hormone is less commonly discussed than T4 and T3, but it plays a role in bone health.

All three hormones require iodine to be made. Iodine is a mineral you get from food — mainly from salt, seafood, and dairy products. Without enough iodine, your thyroid cannot produce these hormones at all.

Key Takeaways

  • T4 and T3 are the main thyroid hormones that control your metabolism — how fast your body burns calories and produces energy.
  • T4 is produced in larger amounts, but T3 is the more active form that cells actually use.
  • Your body converts T4 into T3 as needed, so doctors often measure T4 levels to check thyroid function.
  • Calcitonin is a third thyroid hormone that helps manage calcium in your blood and bones.
  • All thyroid hormones require iodine to be produced, which is why iodine deficiency can cause thyroid problems.

How T4 and T3 work in your body

T4 and T3 act like a gas pedal for your metabolism. They tell your cells to speed up or slow down how much energy they use. This affects everything: how fast your heart beats, how quickly you digest food, how warm you feel, and how much energy you have throughout the day.

T4 is the storage form of thyroid hormone. Your thyroid releases it into the bloodstream, but T4 itself is not very active. Your body converts T4 into T3 in the liver, kidneys, and other tissues. T3 is the form that actually enters cells and does the work. This is why doctors sometimes measure both T4 and T3, and why some people with thyroid problems need both forms of replacement hormone.

The amount of T4 and T3 your thyroid makes is controlled by another gland called the pituitary, which sits at the base of your brain. The pituitary releases a hormone called TSH (thyroid-stimulating hormone) that tells your thyroid when to make more or less hormone. This creates a feedback loop: when thyroid hormone levels drop, TSH rises to signal the thyroid to produce more.

What happens when thyroid hormone levels are wrong

If your thyroid makes too much T4 and T3, you develop hyperthyroidism (overactive thyroid). Your metabolism speeds up, and you may feel anxious, lose weight without trying, sweat more, or have a racing heartbeat. Your body is essentially running too fast.

If your thyroid makes too little T4 and T3, you develop hypothyroidism (underactive thyroid). Your metabolism slows down, and you may feel tired, gain weight, feel cold, or experience dry skin and hair loss. Your body is running too slowly.

Doctors measure the amount of T4 and TSH in your blood to figure out whether your thyroid is working correctly. A blood test can show whether you are making too much, too little, or the right amount of hormone. This is usually the first step when someone has symptoms that might be thyroid-related.

Why iodine matters for thyroid hormone production

Both T4 and T3 are made from the amino acid tyrosine and iodine atoms. T4 contains four iodine atoms, and T3 contains three — that is where the numbers in their names come from. Without iodine, your thyroid cannot build these hormones at all.

In most developed countries, iodine deficiency is rare because table salt is iodized — iodine has been added to it. Seafood, seaweed, and dairy products are also good sources of iodine. Most people get enough iodine without thinking about it. However, in some parts of the world where iodine is not added to salt and seafood is not common, iodine deficiency can cause severe thyroid problems and a condition called goiter, where the thyroid swells.

How calcitonin differs from T4 and T3

Calcitonin is produced by cells in the thyroid called C cells, while T4 and T3 are made by different thyroid cells. Calcitonin helps keep calcium levels in your blood stable by signaling your bones to absorb calcium when blood levels are too high. It is less important than T4 and T3 in day-to-day health, and most people do not notice problems if calcitonin levels are slightly off.

Calcitonin becomes medically important mainly in two situations: when doctors are checking for certain types of thyroid cancer (some cancers produce too much calcitonin), or when someone has severe bone loss and needs treatment to slow it. For most thyroid conditions, doctors focus on T4 and T3 levels instead.

How doctors measure thyroid hormone levels

A simple blood test can measure how much T4, T3, and TSH are in your bloodstream. The most common test measures TSH and free T4 (the T4 that is actually available for your body to use). This combination tells doctors whether your thyroid is producing the right amount of hormone.

If your TSH is high and your T4 is low, it means your thyroid is not making enough hormone — hypothyroidism. If your TSH is low and your T4 is high, it means your thyroid is making too much — hyperthyroidism. Doctors may also measure T3 if they need more detail about your thyroid function, though this is less common.

These blood tests are straightforward and can be done at any lab. Results usually come back within a few days. If your doctor suspects a thyroid problem based on your symptoms, a blood test is almost always the first step to confirm what is happening.

Frequently Asked Questions

What is the difference between T4 and T3?

T4 is the hormone your thyroid releases in larger amounts, but it is not very active on its own. Your body converts T4 into T3, which is the active form that actually works in your cells. T3 is more powerful but produced in smaller quantities. Both are necessary for normal metabolism.

Can you have normal TSH but abnormal T4 or T3?

Yes, though it is less common. Some people have normal TSH but low free T4, or normal TSH but high T3. This can happen with certain thyroid conditions or when someone is taking thyroid replacement hormone. Your doctor may order T4 and T3 tests if your symptoms do not match your TSH level.

Does your thyroid make any other hormones besides T4, T3, and calcitonin?

The thyroid itself makes only these three hormones. However, your thyroid works closely with the pituitary gland, which makes TSH to control the thyroid. The pituitary also makes other hormones that affect different parts of your body, but those are not thyroid hormones.

What happens to thyroid hormone if you do not have enough iodine?

Without iodine, your thyroid cannot make T4 or T3 at all. Your TSH will rise as your pituitary tries to signal the thyroid to produce more hormone, but the thyroid has no raw material to work with. Over time, this can cause the thyroid to swell (goiter) and lead to severe hypothyroidism.

Is T3 replacement better than T4 replacement for hypothyroidism?

Most people do well on T4 replacement alone because the body converts it to T3 as needed. Some people feel better with a combination of T4 and T3, but this is less common. Your doctor can help decide which form works best for you based on your symptoms and blood test results.