Thyroxine is T4. The name refers to the thyroid hormone that contains four iodine atoms, which is why it’s abbreviated with the number 4. The other major thyroid hormone, triiodothyronine, contains three iodine atoms and goes by T3. Together, these two hormones make up what’s collectively called “thyroid hormone.”
Why T4 and T3 Have Different Roles
Your thyroid gland produces far more T4 than T3. T4 is essentially a storage form of thyroid hormone. It circulates through your bloodstream with a relatively long half-life of 5 to 7 days, meaning it lingers in your system and provides a steady reservoir. T3, by contrast, has a half-life of roughly one day and is the more metabolically active form, the one that actually drives cellular processes like metabolism, heart rate, and body temperature.
Think of T4 as the slow-release version. Your body converts it into T3 as needed, right inside your tissues. This conversion happens through specialized enzymes that strip one iodine atom off the T4 molecule, turning it into T3. Two types of these enzymes activate T4 into T3, while a third type deactivates thyroid hormones by converting them into inactive forms. This system lets your body fine-tune thyroid hormone levels in specific tissues without flooding the entire bloodstream with the more potent T3.
How T4 Converts to T3 in Your Body
The conversion from T4 to T3 happens locally in organs like the liver, kidneys, brain, and muscles. One set of converting enzymes primarily controls the T3 levels circulating in your blood, while another set handles local conversion within individual tissues. This means your brain can adjust its own thyroid hormone supply independently of what’s happening in, say, your liver.
Several factors can slow this conversion process. Nutrient deficiencies (particularly selenium, since the converting enzymes are selenium-dependent), chronic stress, severe illness, and certain medications can all reduce how efficiently your body turns T4 into T3. When conversion is impaired, you can have normal T4 levels but still feel symptoms of low thyroid function because your tissues aren’t getting enough active T3.
What T4 Levels Mean on Blood Tests
When your doctor orders thyroid labs, they’ll typically check TSH alongside free T4 (and sometimes free T3). The normal reference range for free T4 in adults is 0.9 to 1.7 ng/dL. A T4 test alone isn’t enough to diagnose a thyroid problem, which is why it’s paired with TSH, the pituitary hormone that tells your thyroid how much hormone to produce.
The relationship between T4 and TSH works like a thermostat. When your T4 drops too low, your pituitary gland increases TSH production to push your thyroid harder. When T4 climbs too high, the pituitary dials TSH back down. So a pattern of high T4 with suppressed TSH points toward hyperthyroidism, while low T4 with elevated TSH suggests hypothyroidism.
T4 and T3 in Thyroid Medications
The most commonly prescribed thyroid medication is levothyroxine, which is a synthetic version of T4. Brand names include Synthroid, Levoxyl, and Tirosint. Because it mimics your body’s natural T4, your tissues can convert it to T3 at their own pace, just as they would with the hormone your thyroid produces on its own. This is why levothyroxine works well for the vast majority of people with hypothyroidism.
A smaller number of patients take liothyronine (brand name Cytomel), which is synthetic T3. This is occasionally prescribed for people who don’t convert T4 to T3 efficiently, or as part of a combination approach with levothyroxine. Because T3 is more potent and clears the body faster, dosing requires more precision than with T4-only therapy.
Why the Distinction Matters
Understanding that thyroxine is T4, not T3, clarifies a lot about how thyroid treatment and testing work. When someone says their “thyroid levels are normal,” they’re usually referring to T4 and TSH. But because T4 is the inactive precursor and T3 is the active hormone doing the work in your cells, some people with normal T4 readings still experience symptoms if their conversion to T3 is suboptimal. This is one reason some clinicians also check free T3 levels, particularly when symptoms persist despite standard treatment.