T3 and T4 are the two main hormones produced by your thyroid gland, a butterfly-shaped organ at the base of your neck. T4 (thyroxine) is the hormone your thyroid makes in the largest quantity, while T3 (triiodothyronine) is the more biologically active form that your cells actually use. Together, they influence every cell and organ in your body, controlling everything from how fast you burn calories to how quickly your heart beats.
What T3 and T4 Do in Your Body
These two hormones act as your body’s metabolic thermostat. They regulate the rate at which you use calories, which directly affects weight gain or weight loss. They speed up or slow down your heart rate, raise or lower your body temperature, and control how quickly food moves through your digestive tract. They also manage how your muscles contract, how fast your body replaces dying skin and bone cells, and even influence your mood.
In infants and children, thyroid hormones are critical for brain development and growth. A deficiency early in life can cause lasting cognitive and developmental problems, which is why newborns are routinely screened for thyroid function shortly after birth.
How Your Thyroid Makes These Hormones
Your thyroid needs two key ingredients to produce T3 and T4: iodine and a protein called thyroglobulin. Thyroid cells actively pull iodine from your bloodstream using a specialized pump on their surface. Inside the gland, an enzyme attaches iodine atoms to the thyroglobulin protein. When two iodine-tagged building blocks join together, the result is either T4 (which contains four iodine atoms) or T3 (which contains three). The finished hormones are then clipped from the protein scaffold and released into your blood, where they hitch a ride on carrier proteins to reach tissues throughout your body.
The thyroid produces far more T4 than T3. Most of the T3 your body uses doesn’t come directly from the thyroid at all. Instead, your liver, kidneys, brain, muscles, and other tissues convert T4 into T3 as needed using specialized enzymes. Think of T4 as a storage form and T3 as the version your cells actually put to work.
How Your Body Keeps Levels in Check
Your brain tightly controls how much T3 and T4 circulate in your blood, keeping them within a narrow range through a feedback loop. It works like this: a region of the brain called the hypothalamus releases a signaling hormone (TRH) that tells the pituitary gland to produce thyroid-stimulating hormone, or TSH. TSH then travels to the thyroid and tells it to ramp up hormone production.
When T3 and T4 levels rise high enough, they signal back to the brain to dial down TRH and TSH production. When levels drop, the brain increases the signal. This is called negative feedback, and it’s the same basic principle as a thermostat cycling your furnace on and off. The hypothalamus sets the “target temperature,” and the system adjusts continuously to stay there.
This is why doctors often check TSH first when evaluating thyroid function. A high TSH typically means your thyroid isn’t making enough hormone and the brain is shouting louder to compensate. A low TSH suggests the opposite: your thyroid is overproducing, so the brain has turned the signal way down.
Free vs. Total Levels on Lab Tests
When you get blood work, you may see results labeled “free T4,” “total T4,” “free T3,” or “total T3.” The distinction matters. Most thyroid hormone in your bloodstream is bound to carrier proteins, making it inactive. Only the small “free” fraction is available for your cells to use. Free T4 and free T3 measurements reflect the hormone that’s actually doing work in your body.
Total T4 and total T3 count both the bound and unbound hormone together. These measurements can be thrown off by anything that changes your carrier protein levels, including pregnancy, estrogen-containing birth control, and certain medications. That’s why free hormone levels are generally more reliable for assessing how your thyroid is actually functioning.
Typical total T4 levels for adults fall roughly in the range of 5 to 12 mcg/dL, though reference ranges vary between labs. Newborns run significantly higher, around 11 to 22 mcg/dL in the first few days of life, then gradually decline through childhood. Your lab report will always list the specific reference range used, so compare your results to that rather than memorizing a single number.
What Happens When Levels Are Too High
When T3 and T4 levels climb above normal, the result is hyperthyroidism. Your metabolism essentially runs too hot. Common symptoms include unintentional weight loss despite increased hunger, a rapid or irregular heartbeat, nervousness and irritability, trembling in the hands, unusual sweating, and fine, brittle hair. Some people notice a visible swelling at the base of the neck where the thyroid sits.
Older adults often experience subtler symptoms that are easy to miss: an irregular heartbeat, unexplained weight loss, depression, or just feeling unusually weak and tired during everyday activities. Because these overlap with so many other conditions, hyperthyroidism in older adults frequently goes undiagnosed for longer.
What Happens When Levels Are Too Low
Low T3 and T4 levels lead to hypothyroidism, where your metabolism slows down. This tends to develop gradually, so symptoms often creep in over months or years. You might notice fatigue, weight gain that’s hard to explain, feeling cold when others are comfortable, constipation, dry skin, thinning hair, and a general mental sluggishness or difficulty concentrating sometimes described as “brain fog.”
Mood changes are common too. Depression, low motivation, and irritability can all stem from an underactive thyroid. Muscle aches, joint stiffness, and a puffy face (especially around the eyes) are other hallmarks. In women, hypothyroidism can cause heavier or irregular periods and difficulty getting pregnant.
Why the T4-to-T3 Conversion Matters
Because most circulating T3 comes from converting T4 outside the thyroid, the conversion process itself can become a bottleneck. The enzymes responsible for this conversion are concentrated in the liver, kidneys, brain, and muscles. Factors that impair their function, including certain nutrient deficiencies (particularly selenium and zinc), chronic illness, calorie restriction, and high stress, can reduce how efficiently your body activates T4 into usable T3.
This is one reason some people with “normal” T4 levels still feel symptomatic. Their thyroid may be producing adequate T4, but their body isn’t converting enough of it into the active T3 form. Checking both free T4 and free T3 together, rather than relying on TSH and T4 alone, can sometimes reveal this mismatch.