T3 and T4 are thyroid hormones that control how fast your cells burn energy. They influence nearly every organ in your body, from your heart and brain to your bones and digestive tract. T4 (thyroxine) is the more abundant form, acting mainly as a reservoir that gets converted into T3 (triiodothyronine), the biologically active version that does most of the heavy lifting at the cellular level.
T4 Is the Raw Material, T3 Is the Active Form
Your thyroid gland releases far more T4 than T3, but T4 is largely a prohormone. It circulates through your bloodstream until enzymes in other tissues strip off one of its iodine atoms, converting it into T3. This conversion happens primarily in the liver, kidneys, and thyroid itself for the T3 that circulates in your blood. But your brain, heart, muscles, and brown fat also convert T4 to T3 locally, right where it’s needed.
A third enzyme does the opposite: it converts T4 into reverse T3 (rT3), a molecule traditionally considered inactive. Reverse T3 can’t activate thyroid hormone receptors the way T3 does. However, it may compete with T3 for receptor binding sites, effectively dampening thyroid hormone activity. During periods of physical stress, chronic inflammation, or serious illness, your body shifts T4 metabolism away from active T3 and toward rT3, which is one reason people often feel sluggish and cold when they’re sick or under prolonged stress.
How They Set Your Metabolic Speed
T3 is a key regulator of mitochondria, the structures inside cells that produce energy. It controls both how many mitochondria your cells build and how hard those mitochondria work. When T3 levels are adequate, your cells efficiently burn carbohydrates, fats, and proteins for fuel. When levels are high, your resting energy expenditure climbs, cholesterol drops, and fat breakdown accelerates. When levels are low, the opposite happens: energy expenditure falls, cholesterol rises, fat breakdown slows, and weight tends to creep up.
In muscle tissue specifically, higher thyroid hormone levels shift muscle fibers from slow-twitch to fast-twitch types. This increases glucose uptake into muscles and ramps up heat production, which partly explains why people with an overactive thyroid feel warm and lose weight, while those with an underactive thyroid feel cold and gain weight.
Keeping Your Body Warm
Body temperature regulation is one of thyroid hormones’ most noticeable jobs. T3 acts on brown fat, a special type of fat tissue whose sole purpose is generating heat. Brown fat produces heat by burning fuel without making usable energy. Instead, the energy is released directly as warmth. T3 controls this process in two ways: it increases the amount of the protein responsible for heat generation in brown fat cells, and it makes brown fat more responsive to signals from the nervous system that trigger warming.
T3 also acts in the brain to raise the body’s temperature set point. Studies in mice show that when T3 is introduced directly into the brain, body temperature rises, but only if brown fat is functional. People with hypothyroidism struggle to maintain their body temperature and often feel cold, while people with hyperthyroidism run warm and become sensitive to heat.
Effects on the Heart and Blood Vessels
Your cardiovascular system is one of the most sensitive targets of thyroid hormones. T3 raises your heart rate, strengthens each contraction of the heart muscle, and relaxes blood vessels to lower resistance in the circulatory system. These three effects work together to increase cardiac output. In people with hyperthyroidism, cardiac output can rise 50 to 300% above normal levels, which is why a racing heart and palpitations are among the most common symptoms of an overactive thyroid.
Paradoxically, this overdrive state actually reduces exercise tolerance. Because the heart is already running close to its maximum at rest, there’s little room to increase performance during physical activity. On the other end, hypothyroidism slows heart rate and weakens contractions, which can cause fatigue and shortness of breath with exertion.
Brain Development and Function
Thyroid hormones are critical for brain development, particularly during fetal growth and early childhood. T3 guides several foundational processes: the migration of neurons to their correct positions, the formation of connections between neurons (synapses), and myelination, the insulation of nerve fibers that allows electrical signals to travel quickly. Disruptions to thyroid hormone supply during these windows can cause lasting neurological problems. Genetic mutations that block thyroid hormone transport into brain cells, for example, lead to severe developmental disorders with pronounced damage to myelination.
In adults, T3 continues to support cognitive function, mood stability, and synaptic flexibility. This is why brain fog, difficulty concentrating, and depression are common complaints in hypothyroidism.
Digestion, Bones, and Other Targets
Thyroid hormones regulate the speed of muscle contractions throughout your digestive tract. In hypothyroidism, reduced motility in the stomach, esophagus, small intestine, and colon leads to constipation, slow gastric emptying, and sometimes heartburn or difficulty swallowing due to weakened esophageal contractions. These symptoms typically reverse with thyroid hormone replacement. Hyperthyroidism, on the other hand, often causes frequent bowel movements, though it can also paradoxically slow gastric emptying by disrupting the normal electrical rhythms of the stomach and upper intestine.
Bones are another target. T3 and T4 regulate the rate of bone turnover, the ongoing cycle of breaking down old bone and building new bone. When thyroid hormone levels are too high for too long, bone breakdown outpaces rebuilding, which can reduce bone density over time. When levels are too low, bone growth and remodeling slow down.
How Your Body Keeps T3 and T4 in Balance
The production of T3 and T4 is controlled by a feedback loop involving three structures. The hypothalamus, a region at the base of the brain, releases a signaling hormone (TRH) that tells the pituitary gland to release thyroid-stimulating hormone (TSH). TSH then travels to the thyroid gland and triggers the release of T4 and T3. When free T3 and T4 levels in the blood rise high enough, they suppress the release of both TRH and TSH, dialing back thyroid hormone production. When levels drop, the brake is released and production ramps up again.
This is why TSH is the standard screening test for thyroid problems. A high TSH usually means the thyroid isn’t producing enough hormone (hypothyroidism), because the pituitary is shouting louder to compensate. A low TSH suggests the thyroid is producing too much (hyperthyroidism), so the pituitary has gone quiet.
What Happens When Levels Are Off
Because T3 and T4 touch so many systems, the symptoms of too much or too little overlap with dozens of other conditions, which is why thyroid disorders often go undiagnosed for months or years.
- Too little (hypothyroidism): weight gain, constipation, slowed heart rate, dry skin and hair, sensitivity to cold, fatigue, brain fog, and depression.
- Too much (hyperthyroidism): weight loss, nervousness, frequent bowel movements, sensitivity to heat, muscle weakness, rapid heart rate, and difficulty sleeping.
Fatigue and an enlarged thyroid gland (goiter) can show up in both conditions, making symptoms alone an unreliable guide. A simple blood test measuring TSH, and sometimes free T4 and T3, is the most reliable way to identify whether your thyroid hormone levels are where they should be.