PTU ↔ Methimazole Conversion Calculator

This calculator converts between propylthiouracil (PTU) and methimazole (MMI) using a selected PTU:MMI ratio. It is intended for pregnancy-related antithyroid drug transitions and provides rounded practical dosing, EMR-ready wording, and a detailed comparison of published conversion approaches.

Important: PTU/MMI conversion is approximate. Published and clinically used ratios vary, and no ratio should be treated as exact pharmacologic equivalence. In pregnancy, the most guideline-supported switch ratio is approximately 20:1 PTU:MMI. Dose adjustment should be based on FT4/TSH trend, symptoms, trimester, adverse-effect risk, fetal considerations, and endocrinology/MFM input when available.

Conversion direction

Current total daily dose, mg/day

Select PTU:MMI ratio

Pregnancy timing

Result

Enter a dose, select a ratio, and click “Calculate Conversion.”

Conversion Notes

20:1 PTU:MMI is the preferred default for this pregnancy calculator because it is the most guideline-supported pregnancy-switch estimate.
10:1 PTU:MMI is included because methimazole labeling states that MMI is at least 10 times as potent as PTU by weight, but clinical comparative data suggest simple 10:1 dosing may not behave as equivalent.
15:1 PTU:MMI is a reasonable intermediate estimate within the 15–20:1 range cited in endocrine review literature.
30:1 PTU:MMI is best presented as a conservative low-MMI estimate rather than a standard equivalence ratio.

Rounding method: Methimazole estimates are rounded to practical 2.5 mg increments. PTU estimates are rounded to practical 50 mg increments. Final prescribing should reflect available tablet strengths, patient-specific disease control, and local practice.

Detailed Linked References

Conversion-ratio references

10:1 potency / comparative-dose basis

DailyMed methimazole label. The label states that methimazole is “at least 10 times as potent as propylthiouracil” on a weight basis, while also noting variability in action. This supports inclusion of 10:1 as a potency-based estimate, but not necessarily as a pregnancy default. DailyMed methimazole label.

He CT, Hsieh AT, Pei D, et al. Comparison of single daily dose of methimazole and propylthiouracil in the treatment of Graves’ hyperthyroidism. Clin Endocrinol (Oxf). 2004;60(6):676-681. PMID: 15163329. PubMed abstract reports that MMI 15 mg/day was more effective than PTU 150 mg/day for induction of euthyroidism, supporting caution that 10:1 may not be clinically equivalent. PubMed.

15–20:1 endocrine review range

Endocrine review basis. Endocrine review literature commonly describes PTU-to-MMI equivalence as approximate and often within a 15–20:1 range. This calculator includes 15:1 as an intermediate estimate rather than as the default pregnancy switch ratio. Use local institutional/endocrinology references if your practice has a preferred conversion table.

20:1 pregnancy guideline basis

Alexander EK, Pearce EN, Brent GA, et al. 2017 Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and the Postpartum. Thyroid. 2017;27(3):315-389. PMID: 28056690. These guidelines are the major ATA pregnancy guidance source for management of thyroid disease in pregnancy and postpartum, including antithyroid drug selection and transition considerations. PubMed | Journal page.

ACOG Practice Bulletin No. 223. Thyroid Disease in Pregnancy. Obstet Gynecol. 2020;135(6):e261-e274. PMID: 32443080. ACOG is commonly cited for the 20:1 PTU:methimazole switch ratio in pregnancy when switching is appropriate. PubMed.

30:1 conservative estimate basis

Clinical teaching / potency range basis. Some clinical teaching material describes MMI as approximately 20–30 times as potent as PTU by milligram dose and gives examples such as PTU 300 mg/day ≈ MMI 10–15 mg/day. This supports including 30:1 only as a conservative low-MMI estimate, not as the primary equivalence.

General antithyroid drug guidance and safety references

Hyperthyroidism guideline

Ross DS, Burch HB, Cooper DS, et al. 2016 American Thyroid Association Guidelines for Diagnosis and Management of Hyperthyroidism and Other Causes of Thyrotoxicosis. Thyroid. 2016;26(10):1343-1421. PMID: 27521067. PubMed.

PTU label safety context

DailyMed propylthiouracil label. PTU labeling includes indications and safety warnings relevant to antithyroid drug selection. DailyMed PTU label.

Disclaimer

This calculator is for educational and clinical documentation support only. It does not replace physician judgment, endocrinology/MFM consultation, local protocols, medication labeling, or individualized patient assessment. Verify drug doses, tablet strengths, contraindications, allergies, hepatic history, and follow-up plan before prescribing.