Thyroid dosage conversion: easy rules you can use

A 25 mcg change matters. If you’re switching between levothyroxine (T4), liothyronine (T3), or natural desiccated thyroid (NDT), simple math plus careful lab checks keep you safe. Below are practical conversion rules, real examples, and tips you can use when talking with your clinician.

Quick conversion rules

Use these as a starting point—not a prescription.

- Units: levothyroxine and thyroid hormones are measured in micrograms (mcg). Tablet strengths for T4 commonly include 25, 50, 75, 88, 100, 125, 150 mcg. T3 pills are often 5 or 25 mcg. NDT is dosed in grains (1 grain = 60 mg).

- T3 vs T4 potency: T3 (liothyronine) is roughly 3–4 times more potent than T4 by microgram. A common clinical rule: 25 mcg T3 ≈ 75–100 mcg T4 (use a 1:3–1:4 ratio). So 5 mcg T3 ≈ 15–20 mcg T4.

- NDT conversion: 1 grain (60 mg) of porcine desiccated thyroid is often treated clinically as roughly equivalent to ~100 mcg levothyroxine. Lab-tested hormone content in a grain is about 38 mcg T4 plus ~9 mcg T3, so product variability exists—monitor labs after any switch.

Practical examples & safety tips

Examples to show how the rules work:

- If you take 100 mcg levothyroxine daily and want to try adding T3, replacing 25 mcg of T4 with 6.25–8.3 mcg T3 is one approach (because 25 mcg T4 ≈ 6–8 mcg T3). Changes like this should be small and supervised.

- Switching from 1 grain NDT to levothyroxine: the common swap used by many providers is 1 grain NDT → ~100 mcg levothyroxine. But because NDT contains active T3, symptoms and labs can shift. Expect to check TSH and symptoms 6–8 weeks after the change.

- If you take 5 mcg T3 alone and want an approximate T4 equivalent, think 15–20 mcg T4. That’s useful when comparing product doses but not for dosing decisions without a clinician.

Safety tips: always test TSH and free T4 (and free T3 if on T3) 6–8 weeks after any dose change. Small numeric swaps add up; a 12.5–25 mcg change in T4 is common practice and often meaningful. If you feel jittery, palpitations, or cold intolerance after a switch, contact your provider right away.

Want a deeper read? Check our guide "Your Guide to Thyroid Hormone Treatments" for details on Synthroid, NDT, and combination therapy. Use these conversion rules to prepare for a clinic visit, then let your clinician adjust doses based on labs and symptoms.