Your thyroid is a small gland in your neck that controls how fast your body uses energy

The thyroid is a butterfly-shaped gland about two inches wide that sits in the front of your neck, just below your Adam's apple. It produces hormones that tell your cells how quickly to burn calories and use oxygen. When your thyroid works normally, you don't notice it — but when it doesn't, nearly every system in your body can feel the effects.

Think of your thyroid as your body's gas pedal. It speeds up your metabolism when you need more energy and slows it down when you need less. The hormones it makes travel through your bloodstream to reach almost every cell, affecting how your heart beats, how warm you feel, how quickly you digest food, and even your mood.

Your thyroid doesn't work alone. Your brain's pituitary gland sends signals telling your thyroid how much hormone to make. Your brain's hypothalamus oversees the pituitary. This three-part system — hypothalamus, pituitary, and thyroid — is called the hypothalamic-pituitary-thyroid axis, and it keeps your metabolism in balance through a feedback loop.

Key Takeaways

  • Your thyroid produces hormones that control how fast your body uses energy and oxygen.
  • The two main thyroid hormones are T4 and T3, and your body converts T4 into T3 as needed.
  • Your brain's pituitary gland sends signals to your thyroid through a hormone called TSH, which tells it how much hormone to produce.
  • When your thyroid makes too much or too little hormone, it affects your heart rate, weight, energy, mood, and body temperature.

The two main hormones your thyroid makes

Your thyroid produces two hormones: T4 (thyroxine) and T3 (triiodothyronine). T4 is the form your thyroid releases directly into your blood. T3 is the more active form that your cells actually use. Your body converts T4 into T3 as needed, so T4 acts like a storage form of thyroid hormone.

Both hormones contain iodine, which is why iodine in your diet matters for thyroid health. If you don't eat enough iodine, your thyroid can't make enough hormone. In countries where salt is iodized, iodine deficiency is rare, but it remains a major cause of thyroid problems worldwide.

Your thyroid also makes a third hormone called calcitonin, which helps regulate calcium levels in your blood. This hormone is less commonly discussed because problems with calcitonin alone are uncommon.

How your brain tells your thyroid what to do

Your pituitary gland produces a hormone called TSH (thyroid-stimulating hormone). TSH is the signal that tells your thyroid how much T4 and T3 to make. When your thyroid hormone levels are low, your pituitary releases more TSH. When levels are high, your pituitary releases less TSH. This feedback loop keeps your thyroid hormone in a narrow, healthy range.

This is why doctors measure TSH when checking thyroid health — it's the most sensitive way to detect whether your thyroid is making the right amount of hormone. A high TSH usually means your thyroid isn't making enough hormone. A low TSH usually means your thyroid is making too much.

Your hypothalamus sits above your pituitary and produces TRH (thyrotropin-releasing hormone), which tells your pituitary to make TSH. Stress, sleep, temperature, and other factors can affect TRH, which is why thyroid hormone levels can shift with life circumstances.

What happens when your thyroid makes too much hormone

When your thyroid produces excess hormone — a condition called hyperthyroidism — your metabolism speeds up. Your heart beats faster, you feel jittery or anxious, you lose weight even when eating normally, you sweat more, and you feel hot when others feel comfortable. You may also feel tired despite the racing feeling, because your body is burning through energy so quickly.

Common causes of hyperthyroidism include Graves' disease (an autoimmune condition where your immune system attacks your thyroid) and thyroiditis (inflammation of the thyroid). Thyroid nodules that produce hormone on their own can also cause excess hormone.

What happens when your thyroid makes too little hormone

When your thyroid produces too little hormone — a condition called hypothyroidism — your metabolism slows down. You feel tired and cold, you gain weight even when eating normally, your digestion slows, your hair becomes dry and brittle, and your mood may drop. Your heart rate slows, and you may feel sluggish in both body and mind.

The most common cause of hypothyroidism in iodine-sufficient countries is Hashimoto's disease, an autoimmune condition where your immune system gradually destroys thyroid tissue. Hypothyroidism can also develop after thyroid surgery, radiation, or certain medications. It becomes more common as you age.

Why your thyroid affects so many parts of your body

Thyroid hormone reaches nearly every cell in your body, so thyroid problems can affect almost any system. Your heart depends on thyroid hormone to beat at the right rate and strength. Your digestive system needs it to move food through your stomach and intestines. Your brain needs it for focus and mood. Your muscles need it for strength. Your skin, hair, and nails all depend on thyroid hormone for healthy growth.

This is why thyroid problems can feel like they're affecting everything at once. A person with an underactive thyroid might experience fatigue, weight gain, constipation, dry skin, hair loss, depression, and brain fog all at the same time — because all of these systems are running slower than normal.

The reverse is true for an overactive thyroid: rapid heartbeat, weight loss, diarrhea, sweating, anxiety, and tremors can all occur together because all systems are running faster than normal.

How doctors test thyroid function

A blood test is the standard way to measure thyroid health. Doctors typically order a TSH test, which measures the signal your pituitary sends to your thyroid. Many doctors also measure free T4, the amount of T4 available for your cells to use. Some order free T3 as well, though this is less common in routine screening.

If your TSH is abnormal, your doctor may also test for thyroid antibodies — proteins your immune system makes if it's attacking your thyroid. This helps identify autoimmune thyroid disease like Hashimoto's or Graves' disease.

An ultrasound of your thyroid may be ordered if your doctor feels a lump or nodule, or if blood tests suggest a structural problem. A thyroid ultrasound shows the size, shape, and texture of your thyroid gland and can detect nodules.

Frequently Asked Questions

Can you feel your thyroid if it's normal?

You may feel the lower part of a normal thyroid gland if you press gently on your neck just below your Adam's apple, but you shouldn't see it or feel a lump. If your thyroid is enlarged (a condition called goiter), it becomes visible or noticeably bumpy. An enlarged thyroid can happen with both too much and too little hormone production.

Does thyroid disease run in families?

Yes. Autoimmune thyroid diseases like Hashimoto's and Graves' disease tend to run in families, and having a family member with thyroid disease increases your risk. Other autoimmune conditions in your family also raise your risk. However, having a family history doesn't mean you will definitely develop thyroid disease.

Can your thyroid change how it works over time?

Yes. Hashimoto's disease typically develops slowly over years as your immune system gradually damages thyroid tissue, so you might have normal thyroid tests one year and abnormal ones later. Graves' disease can also go into remission. Thyroid hormone needs can also change with age, pregnancy, menopause, and other life changes.

What role does iodine play in thyroid health?

Your thyroid needs iodine to make T4 and T3 hormones. Without enough iodine, your thyroid can't produce adequate hormone, leading to hypothyroidism and goiter. In countries with iodized salt, most people get enough iodine. Seaweed, fish, and dairy products are also good sources, though iodine content varies widely.