The thyroid produces hormones that control how fast your body burns energy

Your thyroid is a small gland in your neck that makes two main hormones: T3 and T4. These hormones travel through your bloodstream and tell your cells how quickly to use energy. When your thyroid makes the right amount of hormone, your metabolism—the speed at which you burn calories—stays steady. When it makes too much or too little, that speed changes, and you feel the effects throughout your body.

The thyroid also makes a third hormone called calcitonin, which helps regulate calcium levels in your blood and bones. But the T3 and T4 hormones are what most people notice when something goes wrong with their thyroid.

Key Takeaways

  • Your thyroid produces hormones that control your metabolic rate—how fast your body burns energy at rest and during activity.
  • The pituitary gland in your brain sends signals to your thyroid telling it how much hormone to make, creating a feedback loop that keeps levels stable.
  • Thyroid hormones affect nearly every organ system: your heart rate, body temperature, digestion, mood, and how quickly you gain or lose weight.
  • The thyroid needs iodine from food to make these hormones, which is why iodine deficiency can cause thyroid problems in parts of the world where it is rare in the diet.

How your brain tells your thyroid what to do

Your thyroid does not work alone. Your pituitary gland, a pea-sized structure at the base of your brain, constantly monitors thyroid hormone levels in your blood. When levels drop too low, the pituitary releases a hormone called TSH (thyroid-stimulating hormone) that tells your thyroid to make more. When levels are high enough, the pituitary backs off and releases less TSH.

This feedback loop keeps your thyroid hormone levels in a narrow, stable range. If your thyroid is damaged or not working properly, this system can break down. Your pituitary may keep sending signals to make more hormone, but your thyroid cannot respond—or your thyroid may make too much hormone and the pituitary cannot slow it down fast enough.

What thyroid hormones do to your heart, metabolism, and temperature

Thyroid hormones speed up or slow down almost every process in your body. They tell your heart how fast to beat: too much thyroid hormone and your heart races; too little and it slows. They control how warm you feel by affecting how much heat your cells produce. They also determine how quickly your digestive system moves food through your stomach and intestines.

At the cellular level, thyroid hormones increase how much oxygen your cells use and how much energy they burn. This is why people with an overactive thyroid (hyperthyroidism) often lose weight without trying and feel hot all the time, while people with an underactive thyroid (hypothyroidism) gain weight easily and feel cold.

Your brain, muscles, and bones all depend on the right amount of thyroid hormone. Too little can slow your thinking, weaken your muscles, and make your bones more fragile. Too much can make you anxious, shaky, and unable to sleep.

Why your thyroid needs iodine

Your thyroid cannot make T3 and T4 without iodine, a mineral found in certain foods and in salt. Your body does not make iodine, so you have to get it from what you eat. In countries where iodized salt is common, iodine deficiency is rare. In parts of the world where iodine is scarce in the soil and food, people can develop an enlarged thyroid (goiter) as the gland tries harder and harder to make enough hormone with too little raw material.

Most people in North America and Europe get enough iodine from salt, dairy products, eggs, and seafood. If you avoid salt entirely or eat only non-iodized salt, you may not get enough, though this is uncommon.

How thyroid problems develop

The most common thyroid problems are caused by your immune system attacking the gland. In Hashimoto's thyroiditis, your immune cells damage the thyroid until it cannot make enough hormone. In Graves' disease, your immune system tells the thyroid to make too much. Both are autoimmune conditions and tend to run in families.

Thyroid problems can also develop from damage to the gland itself—from radiation, surgery, or certain medications—or from problems in the pituitary that prevent it from sending the right signals. Some people are born with a thyroid that does not work properly. Pregnancy can trigger thyroid problems in women who are genetically prone to them.

What happens when thyroid hormone levels are off

When your thyroid makes too much hormone (hyperthyroidism), your metabolism speeds up. You may lose weight, feel anxious or irritable, have a racing heartbeat, sweat more than usual, and feel shaky. Your eyes may feel dry or bulge slightly. You may have trouble sleeping or focusing.

When your thyroid makes too little hormone (hypothyroidism), your metabolism slows. You may gain weight, feel tired and cold, have dry skin and hair, feel depressed or foggy, and experience constipation. Your heart rate may drop. These changes can develop slowly over months, so you might not notice them at first.

How doctors measure thyroid function

Doctors check thyroid function with a blood test that measures TSH and often T4 levels. TSH is usually the first test because it is sensitive to even small changes in thyroid hormone. If TSH is abnormal, your doctor will measure T4 to see whether your thyroid itself is the problem or whether the pituitary is sending the wrong signals.

Some doctors also test for antibodies against the thyroid to see whether an autoimmune condition is causing the problem. The results tell your doctor whether your thyroid is working normally, making too much hormone, or making too little.

Frequently Asked Questions

Can stress affect my thyroid?

Stress does not cause thyroid disease, but it can make existing thyroid problems worse. Severe stress may temporarily change how much thyroid hormone your body uses. If you have an autoimmune thyroid condition, stress can trigger a flare-up. Managing stress through sleep, exercise, and relaxation may help, but it cannot fix an underlying thyroid problem.

Does caffeine affect thyroid function?

Caffeine does not damage your thyroid or change how much hormone it makes. However, if you have hyperthyroidism, caffeine can make symptoms like anxiety and a racing heart worse. If you take thyroid medication, caffeine may slightly reduce how much of it your body absorbs, so spacing them apart by a few hours is sometimes recommended.

Can diet change how my thyroid works?

Diet cannot cure thyroid disease, but certain foods contain compounds that affect how your body uses iodine. Cruciferous vegetables like broccoli and cabbage, and soy products, contain goitrogens—substances that can interfere with iodine use in large amounts. Eating normal portions is safe. If you have hypothyroidism and take medication, your doctor may suggest spacing these foods away from your dose.

Is thyroid disease hereditary?

Autoimmune thyroid diseases like Hashimoto's and Graves' run in families, so having a relative with thyroid disease increases your risk. However, having the genetic tendency does not mean you will definitely develop thyroid disease. Environmental triggers like stress, infection, or pregnancy may be needed to activate the condition.