T3 vs T4, what’s the difference?
T4 is the main hormone the thyroid produces, and it is largely a storage and transport form — relatively inactive until converted. T3 is the active hormone that actually drives metabolism in your tissues, and most of it is made by converting T4 in the liver, kidneys, and other tissues rather than by the thyroid itself. That conversion step is why someone can have adequate T4 and still have low active T3, and why testing both is more informative than testing one.
What each hormone does
The thyroid produces roughly 80 percent T4 and 20 percent T3. T4 circulates as a reservoir with a relatively long half-life. T3 is several times more biologically active, binds thyroid receptors in tissue, and has a much shorter half-life.
Functionally, T4 is the supply and T3 is what actually gets used.
Why conversion matters
The conversion of T4 to T3 happens outside the thyroid and can be affected by illness, chronic stress and cortisol, calorie restriction, inflammation, certain medications, and deficiencies in selenium, zinc, or iron.
When conversion is impaired, T4 can look adequate while active T3 is low. Some T4 is also converted into reverse T3, an inactive form that can accumulate under stress or illness and compete with active T3.
What this means for testing and treatment
A panel that includes free T4, free T3, and reverse T3 alongside TSH shows both supply and conversion, which a TSH-only result cannot. This is one reason patients with persistent symptoms and a normal TSH are often left without an explanation.
Standard treatment for hypothyroidism uses T4, which works well for most people. Where symptoms persist despite normalized labs, conversion is one of several things worth examining — with any change to therapy made by your prescribing provider.
More about Thyroid Optimization
Get conversion looked at, not just TSH
Book a consultation with our Folsom naturopathic team for a complete thyroid panel including free T3 and reverse T3.
Bring recent lab work if you have it — we’ll review it with you.
Medical disclaimer: The information on this page is for general education and is not medical advice, diagnosis, or treatment. This page does not diagnose any condition or recommend any specific therapy. Which tests and treatments are appropriate for you depends on your individual history and should be determined by a licensed provider. Do not start, stop, or change any thyroid medication without consulting your prescribing provider.