Thyroid Ratio Calculator
A single TSH rarely tells the whole thyroid story. Ratios between your thyroid labs can add context — how well T4 converts to active T3, how much inactive reverse T3 is around, and how your pituitary-thyroid axis is set. Enter four values from a standard thyroid panel to see all three views at once.
Enter your values above to see your results.
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How it works
The formulas
FT3/rT3 = (Free T3 pg/mL × 100) ÷ reverse T3 ng/dL. FT3/FT4 is computed as a molar ratio: FT3 × 1.536 and FT4 × 12.87 convert both to pmol/L before dividing. TSH Index (Jostel) = ln(TSH) + 0.1345 × FT4 (in pmol/L). All four inputs come from one standard thyroid panel with reverse T3 added.
How reliable are the FT3/rT3 bands?
Honestly: the >20 / 10–20 / <10 bands are functional medicine community conventions, not formal clinical guidelines — no major endocrinology society endorses them. Mainstream guidance has even questioned the clinical utility of routine reverse T3 testing. We show the ratio because practitioners ask for it, and we label the evidence level plainly rather than dress it up.
What the other two views add
The FT3/FT4 ratio has published population data: in a study of over 5,600 subjects, healthy controls showed a median of ~0.32 (rough interval 0.20–0.50), and a subset of patients on T4-only therapy ran below range despite normal TSH. Jostel's TSH Index quantifies the pituitary's response to T4 — a low index despite low-normal T4 has been studied as a clue to central thyroid axis problems, and is referenced in the 2018 European Thyroid Association guidance.
Limitations
Ratios compress complex physiology into single numbers. Assay differences between labs, acute illness, medications (including thyroid hormone), and fasting state all move these values. None of these views diagnoses anything — they are conversation starters for your next lab review.
Frequently asked questions
What is a good Free T3 to Reverse T3 ratio?
Functional medicine practitioners commonly look for a ratio above 20 (with FT3 in pg/mL and rT3 in ng/dL). Be aware this is a community convention, not a formal guideline — and mainstream endocrinology has questioned routine rT3 testing altogether. Use it as one signal among many.
What does the Free T3 to Free T4 ratio tell me?
It approximates how much active T3 you have relative to the T4 prohormone, which is influenced by conversion. In published research, healthy controls cluster around a median of ~0.32 (molar ratio), with a rough interval of 0.20–0.50.
What is the TSH Index?
Jostel's TSH Index combines TSH and Free T4 into one number describing pituitary-thyroid axis balance: ln(TSH) + 0.1345 × FT4(pmol/L). The proposed reference interval is 1.3–4.1. It has been studied mainly as an aid for detecting central (pituitary-level) thyroid dysfunction.
My labs use different units — can I still use this?
Enter values in the units shown (pg/mL, ng/dL, mU/L) — these are the standard US lab units. If your report shows pmol/L, divide Free T3 by 1.536 and Free T4 by 12.87 to get pg/mL and ng/dL respectively.
Can these ratios diagnose a thyroid condition?
No. Every ratio here is a screening signal at best — and one of them (FT3/rT3) rests on community convention rather than formal evidence. Diagnosis requires a clinician interpreting your full panel, symptoms, and history.
Sources
- Gullo D, et al. Levothyroxine-treated patients with normal TSH show impaired T3/T4 ratio: evidence from a large cohort. PLoS One, 2011.
- Jostel A, Ryder WD, Shalet SM. The use of thyroid function tests in the diagnosis of hypopituitarism: definition and evaluation of the TSH Index. Clin Endocrinol (Oxf), 2009.
- Schmidt RL, et al. Reverse T3 testing: an analysis of clinical utility using national reference laboratory data. Ann Clin Lab Sci, 2020.
This calculator is for informational purposes only and is not medical advice, diagnosis, or treatment. Reference ranges are general population guidelines — always discuss your lab results with a qualified healthcare professional.