If you’ve been told your thyroid meds aren’t fixing everything, you probably heard about a T4 / T3 combo. It sounds technical, but the idea is simple: give your body both main thyroid hormones so it can work better.
T4 (levothyroxine) is the synthetic version of the hormone thyroxine that most doctors start you on. Your body normally changes T4 into T3 (liothyronine), which is the active form that actually powers metabolism. Some people don’t convert enough, so they stay tired or cold even on a normal dose. Adding a small amount of synthetic T3 fills that gap.
The combo usually means a standard dose of levothyroxine plus a tiny tablet of liothyronine taken once daily (often in the morning). The goal is to keep blood‑test numbers steady while you feel more energetic.
Doctors look at two main clues: persistent symptoms and lab results. If you still feel sluggish, have brain fog, or can’t lose weight after several months on T4 alone, they may check your free‑T3 level. Low free‑T3 with a normal TSH often triggers the combo.
Another reason is specific conditions that affect conversion, like adrenal fatigue, chronic stress, or certain gut issues. In those cases, adding T3 can smooth out the hormone roller coaster.
The typical starting point is 12‑25 µg of liothyronine on top of your regular levothyroxine dose. Doctors usually start low because T3 works fast and can cause jitters if you take too much.
After a few weeks, they’ll recheck labs – especially TSH, free‑T4 and free‑T3 – and adjust the numbers. The key is regular follow‑up; the right balance can change from month to month.
Side effects are easy to spot: rapid heartbeat, anxiety, insomnia, or shaky hands usually mean the dose is too high. If that happens, cut back the T3 and let your body settle before trying again.
It’s also smart to take the T3 dose on an empty stomach, about 30 minutes before breakfast. Food can lower its absorption, making it harder to gauge the right amount.
People who are pregnant, have heart disease, or take certain antidepressants need extra caution. Always tell your pharmacist and doctor about any other meds you’re using.
The bottom line? T4 / T3 therapy can turn a “still not great” feeling into a smoother day, but only when it’s tailored to your labs and symptoms. If you think you might fit the profile, bring up the combo at your next appointment – ask for a free‑T3 test and see if a small T3 add‑on could help.
Remember, thyroid health isn’t one‑size‑fits‑all. Tracking how you feel, keeping labs current, and staying in touch with your doctor are the best ways to make sure the therapy works for you.
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