Thyroid conversion dosing — quick, practical rules you can use

Switching between levothyroxine (T4), liothyronine (T3), and desiccated thyroid can feel confusing. Micrograms, grains, and different T4:T3 mixes make it messy. Here are clear, commonly used conversion points and sensible rules for changing doses without causing surprises.

Common dose equivalents

Use these as starting estimates only — labs and symptoms must guide real adjustments.

• General T4:T3 potency: 1 mcg of T3 is roughly equal to 3–4 mcg of T4. So 25 mcg T3 often acts like about 75–100 mcg T4.
• Desiccated (Armour) rough equivalence: 1 grain (60 mg) of desiccated thyroid is commonly treated as roughly equivalent to around 100 mcg levothyroxine. Formulations vary, so treat this as an estimate.
• When switching from combination products (T4+T3) to T4 alone, expect symptoms to change because T3 acts faster and is shorter-acting. Dose reductions or split dosing might be needed.

Those rules help plan a switch, but they are not exact. Individual response varies with age, weight, other meds, and how well you absorb the pill.

Practical dosing tips & monitoring

Make one change at a time. Change a single drug or dose and wait 6 weeks before checking TSH — that’s how long TSH usually takes to settle after changing T4. If you added T3, remember TSH is less reliable for short-term T3 effects; check free T4 and free T3 and watch symptoms.

Start low if you’re older or have heart disease. Thyroid hormones speed up the heart. If you’re over 60 or have coronary disease, many clinicians raise doses slowly — often by no more than 12.5–25 mcg of levothyroxine every 6–8 weeks.

Split T3 dosing if using liothyronine. T3 peaks and drops faster than T4, so many patients do better taking small T3 doses twice daily rather than one large morning dose.

Pregnancy and weight changes matter. Pregnancy typically increases levothyroxine needs; many women need about a 25–30% higher dose. Big weight changes can change dose needs too.

Watch interactions. Take levothyroxine on an empty stomach and avoid calcium, iron, and some supplements within 4 hours. Certain drugs and gut issues can change how much hormone enters your bloodstream.

Symptoms matter. Lab numbers guide you, but if you feel extreme fatigue, palpitations, or anxiety after a switch, contact your clinician quickly. Small numeric changes can matter more if you feel poorly.

Final note: use these conversions only as a starting plan. Always confirm with blood tests and work with your clinician to tailor dose, timing, and follow-up. If you want, I can make a simple conversion example for your current doses — tell me what you take now.