T4 and T3: What You Need to Know

Thyroid hormones T4 (thyroxine) and T3 (triiodothyronine) control your energy, mood, and metabolism. Most of the hormone made by your thyroid gland is T4, which acts as a storage form. T3 is the active hormone that tells your cells to speed up or slow down. The body converts T4 into T3 mostly in the liver and muscles, so both hormones matter for normal function.

Doctors usually measure TSH, free T4 and sometimes free T3 to check thyroid health. TSH tells whether the pituitary thinks your thyroid output is enough. A high TSH often means underactive thyroid; a low TSH can mean it’s overactive. Free T4 gives a picture of available T4, while free T3 helps when symptoms don’t match TSH and T4 results.

T4 vs T3: Key differences

T4 is stable and stays in blood longer; that’s why most replacement drugs are T4-only, like levothyroxine. T3 acts quickly and peaks faster, so liothyronine (synthetic T3) can cause energy spikes and needs careful dosing. Some people feel better on a combination of T4 and T3, but evidence is mixed and doctors monitor closely when T3 is used.

If you take thyroid medicine, timing matters. Take levothyroxine on an empty stomach, at least 30 to 60 minutes before breakfast, for consistent absorption. Calcium, iron, and some supplements or antacids can block absorption, so separate them by several hours. Birth control pills and some drugs affect thyroid tests, so tell your provider about any medicines you use.

When labs and symptoms don’t match

It’s common for symptoms like fatigue, weight changes, or hair thinning to persist even when labs look normal. Reasons include poor conversion of T4 to T3, other hormonal issues, or non-thyroid causes like sleep apnea or iron deficiency. If you still feel unwell, ask your clinician to check free T3, ferritin, vitamin D and adrenal symptoms, and review medications that interfere with thyroid function.

For suspected low T3 specifically, clinicians may consider liothyronine trials in low doses for a short period. Any T3 treatment needs careful follow-up because it can raise heart rate and affect bone health. Don’t adjust doses on your own; small changes can have big effects.

Keep track of symptoms, medication timing, and blood test dates. A simple notebook or phone app helps you spot patterns and gives your doctor clearer information. If you’re pregnant or trying to conceive, thyroid levels need stricter control, so communicate with your provider promptly.

Questions about lab values, drug interactions, or feeling worse despite treatment are valid. Use this page as a starting point and talk to a clinician who knows your full history for tailored advice.

Before testing, avoid changing medication timing or starting supplements right before labs. Try to have blood drawn the same time of day and tell the lab staff if you took your pill. Repeat tests after dose changes or pregnancy. Keep copies of prior tests to compare trends; trends matter more than single numbers and clinical context.