Why you're still tired on levothyroxine: T4 to T3 conversion

How does your body convert T4 to T3?

Your thyroid releases both T4 and T3, but mostly T4—about 80%. The rest of the conversion happens in your liver, kidneys, and other tissues, through enzymes called deiodinases that strip iodine atoms off T4 to make it active.

Levothyroxine mimics what your thyroid would have released: it's synthetic T4. Once you absorb it (which depends on timing, stomach acid, and other medications), your body has to do the conversion work. If that process stalls, you have plenty of T4 circulating but not enough T3 where it matters—your cells.

T4 is a storage form; T3 is the active hormone your cells use. Conversion is the bridge.

What actually affects T4-to-T3 conversion?

The deiodinase enzymes that do the conversion need several things to work: adequate selenium, iron, zinc, and iodine; stable cortisol; a functioning liver and kidneys; and normal body temperature. Stress, calorie restriction, inflammation, and certain medications (beta-blockers, propranolol, amiodarone, some anticonvulsants) can slow it down. So can high reverse T3, which is a metabolic brake your body produces when it's under threat.

Chronic illness, autoimmune disease, and poorly controlled blood sugar also reduce conversion efficiency. Pregnancy and estrogen-based contraceptives can change how much thyroid hormone your body needs and how it processes it. None of these are thyroid problems themselves—they're conversion problems, and they're worth investigating with your doctor.

• Selenium, iron, and zinc deficiency
• Chronic stress and elevated cortisol
• Liver or kidney dysfunction
• High reverse T3 (metabolic adaptation)
• Poorly controlled blood sugar and insulin resistance
• Certain medications (beta-blockers, amiodarone, some anticonvulsants)
• Estrogen-based contraceptives or HRT

Conversion isn't just about thyroid—it's about your whole metabolic picture.

Can you test whether conversion is the problem?

Yes. A full thyroid panel that includes free T4, free T3, and reverse T3 can show you whether you have plenty of T4 but low or low-normal T3. If your free T4 is high-normal or high and your free T3 is low-normal or low, conversion is likely part of your picture. That's the conversation to bring to your appointment.

Your doctor can also order tests for the nutritional and metabolic factors above—ferritin, B12, selenium (though this is less commonly measured), fasting glucose, and cortisol patterns. The point isn't to self-diagnose; it's to gather data so your clinician can see the whole picture.

What does the evidence say about T4-only vs. combination therapy?

This is where honest science looks less clean than marketing does. Most guidelines—including the American Thyroid Association—support T4 monotherapy as first-line treatment for hypothyroidism, because it's well-studied, stable, and works for most people. The trials show that a dose of T4 that brings TSH into range also normalizes T3 in most patients.

But a real minority of people don't feel well on T4 alone even with 'normal' labs. Some research suggests that combination T4 plus T3 therapy (liothyronine, or synthetic T3) improves fatigue and mood in certain subgroups—particularly people with persistent symptoms despite adequate T4 dosing. Other studies show no difference. The variation is real, and it's not yet clear which patients will benefit or why.

This is not a failure of T4 therapy; it's a frontier of individual variation. If you've optimized your T4 dose, addressed conversion factors, and still feel unwell, a conversation with your doctor about whether a trial of combination therapy makes sense for you is reasonable. It's not standard, but it's not fringe either.

What should you do at your next appointment?

Bring a list: your current levothyroxine dose, when you take it, what you take it with, how long you've been on it, and how you feel. Bring your recent labs if you have them. Ask whether your doctor has checked free T3 and reverse T3 alongside your TSH and free T4—if not, that's a reasonable request.

Ask about conversion factors: Do you have nutritional deficiencies? Is your cortisol or blood sugar dysregulated? Are your medications interfering? Are you under prolonged stress? These aren't 'alternative' questions; they're metabolic medicine. If your conversion is compromised, no adjustment to your T4 dose will fix it until you address the root.

If your labs show good T4 but low T3, and you've addressed the conversion factors above, a discussion about whether a trial of combination therapy or a different formulation is worth exploring is fair. Your doctor may say no, and that's okay—but the conversation itself is evidence-based.

The bottom line

Feeling tired on levothyroxine doesn't mean the medication is wrong or that your doctor is missing something. It usually means your conversion pathway needs support—and that's fixable. Start with a full panel, look at the metabolic and nutritional factors that affect conversion, and bring that data to your clinician. You deserve to feel well, and the science to get you there exists.