The thyroid is part of the endocrine system, the network of glands throughout your body that produce hormones. It sits at the front of your neck, just under the skin, and its primary job is making hormones that control your metabolism. But while the thyroid officially belongs to the endocrine system, its influence reaches far beyond that single system, affecting your heart, bones, brain, and body temperature.
The Endocrine System and What the Thyroid Does in It
The endocrine system is made up of glands that release hormones directly into your bloodstream. These hormones act as chemical messengers, telling organs and tissues what to do and how fast to do it. Other glands in this system include the pituitary gland (in the brain), the adrenal glands (above the kidneys), and the pancreas.
The thyroid’s specific role within this system is regulating metabolism, the process your body uses to convert food into energy. It does this by releasing two main hormones: T4 (thyroxine) and T3 (triiodothyronine). About 80% of what the thyroid releases is T4, with the remaining 20% being T3. T3 is the more active form, and your body converts T4 into T3 as needed. The thyroid also produces a third hormone, calcitonin, which helps regulate calcium levels in your blood.
How Your Brain Controls the Thyroid
The thyroid doesn’t operate independently. It takes orders from the brain through a feedback loop involving two other structures: the hypothalamus and the pituitary gland, both located in your brain. The hypothalamus sends a signal to the pituitary, which then releases thyroid-stimulating hormone (TSH) into the bloodstream. TSH tells the thyroid how much hormone to produce.
When thyroid hormone levels in the blood rise high enough, they signal back to the brain to reduce TSH output. When levels drop, the brain ramps TSH back up. This is called negative feedback, and it works like a thermostat, keeping hormone levels in a narrow range. Even a brief signal from the hypothalamus can trigger a prolonged release of TSH from the pituitary, which makes the system highly responsive to small shifts.
How Thyroid Hormones Affect Other Body Systems
Although the thyroid belongs to the endocrine system, its hormones act on virtually every organ system. That’s why thyroid problems tend to cause symptoms that seem unrelated to one another.
Heart and Circulation
Thyroid hormones directly influence heart rate and blood vessel function. Low thyroid hormone levels raise cholesterol, which increases the risk of heart disease and stroke over time. High thyroid hormone levels can trigger an irregular heart rhythm called atrial fibrillation, and they also change blood chemistry in ways that make clotting more likely.
Metabolism and Body Temperature
Thyroid hormones set the pace of your basal metabolic rate, the amount of energy your body burns at rest. They do this partly by influencing how your cells use oxygen and partly by activating brown fat, a specialized tissue that generates heat. During cold exposure, your nervous system works with the thyroid to boost heat production in brown fat, protecting against hypothermia. When thyroid hormone levels are too high, the body burns through energy excessively, leading to weight loss, sweating, and heat intolerance. Too little, and metabolism slows, causing weight gain and cold sensitivity.
Brain and Development
In infants, thyroid hormone is critical for brain development. In adults, it continues to influence mood, concentration, and energy levels. This is why both an underactive and overactive thyroid commonly cause cognitive symptoms like brain fog, anxiety, or depression.
Muscles and Bones
Thyroid hormones affect muscle fiber composition and energy use in muscle tissue. High levels shift muscle fibers toward a type that burns energy faster, which increases overall energy expenditure but can cause muscle weakness over time.
Iodine: The Nutrient Your Thyroid Needs Most
Iodine is a building block of both T3 and T4. Your body can’t make iodine on its own, so it has to come from food. Adults need about 150 micrograms per day, roughly the amount in half a teaspoon of iodized salt. Pregnant women need more (220 mcg), and breastfeeding women need the most (290 mcg), because the developing baby depends on maternal thyroid hormones for brain growth.
In countries where iodized salt is common, severe deficiency is rare. But mild deficiency still occurs and can cause the thyroid to enlarge (a condition called goiter) as it works harder to pull iodine from the blood. Good dietary sources include seafood, dairy, eggs, and iodized table salt.
What Happens When the Thyroid Malfunctions
Because thyroid hormones touch so many systems, problems with the gland produce wide-ranging symptoms. The two most common conditions are hypothyroidism (underactive thyroid) and hyperthyroidism (overactive thyroid).
With hypothyroidism, the thyroid produces too little hormone. You might notice fatigue, weight gain, dry skin, constipation, and feeling unusually cold. It’s far more common than hyperthyroidism and often results from the immune system attacking the thyroid.
With hyperthyroidism, the thyroid overproduces hormone. Symptoms tend to be the opposite: rapid heart rate, unintended weight loss, anxiety, tremors, and heat intolerance. Left untreated, hyperthyroidism raises the risk of serious heart complications.
Doctors typically check thyroid function with a blood test measuring TSH. In a healthy person, TSH falls between roughly 0.45 and 4.12 mIU/L. A high TSH suggests the thyroid is underperforming (the brain is sending louder signals to compensate), while a very low TSH suggests it’s overproducing. During pregnancy, the expected range shifts lower, with an upper limit closer to 2.5 mIU/L in the first trimester.