T3 T4 Ratio: What It Really Tells You About Your Thyroid

If you've seen both T3 and T4 on your lab report, the ratio between them can add useful context. T4 (thyroxine) is produced mainly by the thyroid and acts as a reservoir. T3 (triiodothyronine) is the active form your body uses. The T3/T4 ratio—usually calculated from free T3 and free T4—helps show how much of the stored hormone is being converted into the active form.

How the ratio is measured and what’s normal

Labs report results in different units, so the exact number varies. A practical approach: use free T3 (pg/mL) divided by free T4 (ng/dL). Typical lab ranges are roughly free T3 2.3–4.2 pg/mL and free T4 0.8–1.8 ng/dL. That often gives a ratio in the low single digits—many healthy people fall around 2–4. Don’t treat one number as absolute: methods and units differ between labs, so compare trends and units rather than a single value.

What a high or low ratio might mean

High ratio: when T3 is relatively higher than T4 it can mean increased conversion to active hormone. That happens in some cases of hyperthyroidism or isolated T3 toxicosis. You might feel jittery, have fast heart rate, weight loss, or heat intolerance.

Low ratio: if T3 is low compared with T4, conversion may be impaired. Reasons include non-thyroidal illness ("low T3 syndrome"), certain medications, chronic stress, or problems with deiodinase enzymes. Symptoms often look like hypothyroidism—fatigue, weight gain, cold sensitivity—but labs can be mixed.

Also remember total hormones can be misleading: binding proteins (pregnancy, estrogen, medications) change total T3/T4 but not free levels. That’s why free T3/free T4 is preferred for ratio checks.

Practical tips: get a full thyroid panel—not just one hormone. Ask for TSH, free T4, free T3, and if needed reverse T3 and thyroid antibodies (anti-TPO, anti-Tg). Take labs consistently (same time of day), stop high-dose biotin 48–72 hours before testing, and tell your clinician about meds like amiodarone, steroids, or supplements that affect conversion.

What to do if your ratio is off: don’t panic. Match the lab with how you feel. If you have symptoms that fit hyper- or hypothyroidism, bring your results to an endocrinologist or your doctor. They’ll look at trends, not a single number, and may repeat tests or adjust meds. If you're already on thyroid treatment and feel worse despite a "normal" TSH, asking about free T3 and the ratio can be useful.

Want a clear next step? Save copies of your labs, note symptoms and their timeline, and discuss both with your clinician. The T3/T4 ratio is a tool—not the whole story—but used the right way it can point you and your doctor to smarter testing and better care.