T3, or triiodothyronine, is the most active thyroid hormone in your body. It regulates how fast your cells burn energy, how your heart beats, how you maintain body temperature, and how your brain functions. While your thyroid gland produces some T3 directly, most of it is created when other tissues convert the less active hormone T4 into T3. Nearly every organ in your body responds to T3, which is why even small shifts in its levels can produce wide-ranging symptoms.
How Your Body Produces T3
Your thyroid gland releases mostly T4, a precursor hormone that acts as a reservoir. Specialized enzymes then strip an iodine atom from T4 to create T3, the form your cells actually use. This conversion happens primarily in the liver, kidneys, and thyroid itself through one type of enzyme, and in the brain and muscles through another. In the brain specifically, this second enzyme is responsible for roughly 80% of the T3 found in the cerebral cortex, which explains why brain function is so sensitive to thyroid status.
A third enzyme does the opposite job: it inactivates T3 and T4 by converting them into forms your body can’t use. This system of activating and deactivating enzymes gives your tissues precise local control over how much T3 is available at any given moment, independent of what’s circulating in your blood.
T3 and Your Metabolism
T3 is the primary driver of your basal metabolic rate, the energy your body burns just to stay alive at rest. It does this by increasing the production of ATP, the molecule cells use as fuel, and by maintaining the electrical gradients across cell membranes that power everything from nerve signaling to muscle contraction. Two gradients are especially important: the sodium/potassium balance across cell walls and the calcium balance inside muscle cells. Keeping these gradients running is energy-intensive work, and T3 keeps it going.
T3 also makes energy production deliberately “leaky.” In skeletal muscle, it increases the rate at which protons slip through the inner membrane of mitochondria, your cells’ power plants. This leak means mitochondria have to burn more fuel to keep up with ATP demand, and the excess energy is released as heat. This is one reason people with an overactive thyroid often feel warm and lose weight, while those with an underactive thyroid feel cold and gain weight easily.
How T3 Affects Your Heart
Your cardiovascular system is one of the most responsive targets of T3. The hormone increases heart rate and strengthens the force of each contraction, boosting the volume of blood your heart pumps per minute. At the same time, it relaxes blood vessel walls, lowering the resistance blood encounters as it flows through your body. The net result is higher cardiac output with less strain on arterial walls.
T3 appears to directly influence cardiac pacemaker cells, the cluster of tissue that sets your heart’s rhythm. When T3 levels drop below normal, cardiac output, heart rate, stroke volume, and the strength of heart contractions all decrease. This is why fatigue and exercise intolerance are hallmark symptoms of hypothyroidism, and why a racing heart is common in hyperthyroidism.
Body Temperature Regulation
T3 is central to how your body generates and maintains heat. One of its most important roles is in brown adipose tissue, a type of fat whose sole purpose is producing warmth. T3 activates a protein in brown fat cells called UCP1 that uncouples energy production from ATP synthesis, converting calories directly into heat instead. Without adequate T3, the thermogenic capacity of brown fat drops substantially. Clinically, this shows up as the warm skin and heat intolerance of hyperthyroidism or the cold hands, feet, and overall chill of hypothyroidism.
T3 also amplifies heat production indirectly by stimulating the sympathetic nervous system, your “fight or flight” wiring, which in turn ramps up further T4-to-T3 conversion inside brown fat and boosts UCP1 expression even more. This creates a feedback loop where T3 reinforces its own heat-generating effects.
Brain Function, Mood, and Neurotransmitters
T3 plays a significant role in brain chemistry. It influences the levels of three key signaling molecules: serotonin, dopamine, and norepinephrine. These neurotransmitters collectively regulate mood, motivation, learning, and memory. In humans, serotonin levels in the blood correlate positively with T3 concentrations, rising during hyperthyroidism and falling during hypothyroidism.
Animal research shows just how dramatic this relationship can be. When thyroid hormone was depleted, brain levels of norepinephrine dropped to near zero, dopamine plummeted, and serotonin fell by about a third. Supplementing with T3 restored all three neurotransmitters close to normal. This connection helps explain the depression, brain fog, and difficulty concentrating that often accompany low thyroid function, as well as the anxiety and irritability that can come with excess T3.
T3 also supports the brain’s ability to form long-term memories. Hypothyroidism in adults impairs a process called long-term potentiation, which is the mechanism by which neurons strengthen connections during learning. Both animal studies and human observations confirm that low thyroid states lead to measurable deficits in learning and memory.
Cholesterol and Fat Metabolism
T3 directly lowers LDL cholesterol, the type most strongly linked to cardiovascular disease. It does this by increasing the number of LDL receptors on liver cells, which pulls more LDL particles out of the bloodstream and into the liver for breakdown. T3 also reduces LDL oxidation, the chemical modification that makes LDL particles more damaging to artery walls. This is why people with untreated hypothyroidism frequently have elevated cholesterol that improves once thyroid levels are corrected.
Bone Remodeling
Your bones are constantly being broken down and rebuilt in a cycle called remodeling, and T3 regulates the speed of that cycle. During childhood and skeletal development, T3 has an anabolic effect, promoting bone growth. In adults, however, excess T3 shifts the balance toward bone breakdown. Hyperthyroidism shortens the remodeling cycle and tips it in favor of resorption, meaning bone is removed faster than it’s replaced. Over time, this creates a high-turnover state that can reduce bone density and increase fracture risk.
Digestion and Gut Motility
T3 controls how quickly food moves through your digestive tract. It activates the muscular contractions of the upper gastrointestinal system, increasing both the frequency and strength of the wave-like movements that push food along. In hypothyroidism, these contractions become less frequent and weaker, which is why constipation and reduced appetite are among the most common digestive complaints. In hyperthyroidism, the opposite happens: intestinal transit speeds up, often causing diarrhea and increased appetite. At very high thyroid hormone levels, the gut can produce abnormally strong contractions that travel in both directions, mimicking the motor patterns seen in vomiting and diarrhea.
How T3 Levels Are Kept in Check
Your body regulates T3 through a negative feedback loop involving the hypothalamus and pituitary gland. When T3 levels rise above normal, the hypothalamus reduces its production of thyrotropin-releasing hormone (TRH), which in turn tells the pituitary to release less thyroid-stimulating hormone (TSH). Less TSH means the thyroid produces less hormone, bringing levels back down. When T3 drops, the process reverses.
Interestingly, this system is not symmetrical. Research shows that only T3 levels elevated into the above-normal range are capable of fully suppressing TRH production back to baseline. This means the brain is calibrated to be more sensitive to drops in thyroid hormone than to mild elevations, a design that prioritizes protecting against hypothyroidism.
Normal T3 Levels
If your doctor orders T3 testing, you’ll typically see two possible measurements. Total T3, which includes both the protein-bound and free forms, normally falls between 79 and 165 nanograms per deciliter in adults. Free T3, the unbound fraction that is biologically active, ranges from 2.3 to 4.1 picograms per milliliter. Free T3 is generally considered the more useful number because it reflects the hormone actually available to your cells, though reference ranges can vary slightly between laboratories.