TSH & FT3:FT4 Ratio Note Calculator
Read where your TSH, free T4 and free T3 sit against typical or your own lab’s reference ranges, convert between ng/dL, pg/mL and pmol/L, see the combined TSH–FT4 pattern and the FT3:FT4 ratio in both unit systems, plus optional Jostel TSH index and SPINA research indices — a note, not a diagnosis.
How it works
Each result vs its reference range (low / within / high) · free T4 pmol/L = ng/dL × 12.87 · free T3 pmol/L = pg/mL × 1.536 · FT3:FT4 ratio = FT3 (pg/mL) ÷ FT4 (ng/dL), SI ratio = FT3 (pmol/L) ÷ FT4 (pmol/L) · position in range = (value − low) ÷ (high − low) × 100% · Jostel TSHI = ln(TSH) + 0.1345 × FT4 (pmol/L)
TSH is the pituitary’s feedback signal: it rises when thyroid hormone is low (hypothyroid pattern) and falls when it is high (hyperthyroid pattern), so its position against the reference range is the usual first read. Free T4 is the main hormone the thyroid releases, and free T3 the active form made from it. The calculator accepts each value in the unit on your report — ng/dL or pmol/L for free T4, pg/mL or pmol/L for free T3 — converts between them, and compares every value with typical adult ranges, an approximate first-trimester range or the ranges you copy from your own lab report. It then names the combined TSH–free T4 pattern (for example high TSH with normal free T4, the pattern called subclinical hypothyroidism), shows where each value sits within its range and gives the free T3 to free T4 ratio in both unit systems. The ratio and the optional research indices (Jostel TSH index, SPINA-GT and SPINA-GD) describe thyroid balance but are not validated diagnostic cut-offs. Reference ranges differ between laboratories and assays, and only a clinician can interpret results together with symptoms, medicines and history.
Worked example
With TSH 2.0 mIU/L, free T4 1.2 ng/dL and free T3 3.1 pg/mL: TSH sits inside the 0.4–4.0 range, free T4 is 1.2 × 12.87 = 15.4 pmol/L (within 0.8–1.8 ng/dL) and free T3 is 3.1 × 1.536 = 4.76 pmol/L (within 2.3–4.2 pg/mL), so all results are in range. The FT3:FT4 ratio is 3.1 ÷ 1.2 ≈ 2.58 in conventional units, or 4.76 ÷ 15.44 ≈ 0.308 in SI units — a note on balance, not a diagnosis.
Sources: American Thyroid Association — thyroid function tests · Alexander EK et al. 2017 ATA guidelines for thyroid disease during pregnancy (Thyroid 2017;27:315–389) · Jostel A, Ryder WD, Shalet SM. Clin Endocrinol 2009 — the TSH index (PubMed) · Dietrich JW et al. Front Endocrinol 2016 — calculated parameters of thyroid homeostasis (PubMed)
Frequently asked questions
What is a normal TSH?
Most laboratories use roughly 0.4–4.0 mIU/L for adults, though some use narrower or wider ranges, TSH tends to run higher in older people, and pregnancy has its own targets. Choose “Enter my lab’s ranges” and copy the range from your own report for the most accurate reading.
How do I convert free T4 and free T3 between units?
Free T4: multiply ng/dL by 12.87 to get pmol/L (1.2 ng/dL ≈ 15.4 pmol/L), or divide pmol/L by 12.87. Free T3: multiply pg/mL by 1.536 to get pmol/L (3.1 pg/mL ≈ 4.76 pmol/L). TSH in mIU/L and µIU/mL is the same number. Pick the unit next to each field and the calculator converts for you.
What does the FT3:FT4 ratio mean?
It is a rough marker of how much active T3 you have relative to T4. In conventional units (pg/mL over ng/dL) values around 2–4 are common; in SI units (pmol/L over pmol/L) the same ratio is about 0.12 times smaller. There is no universal healthy value, and it is used descriptively rather than to diagnose.
What do the combined patterns mean?
Doctors read TSH and free T4 together. High TSH with low free T4 is the classic pattern of an underactive thyroid, low TSH with high free T4 that of an overactive thyroid, and an abnormal TSH with normal free T4 is called “subclinical”. Mismatched results can come from illness, medicines, biotin or timing of thyroxine, so they are often repeated.
Can this diagnose a thyroid disorder?
No. Thyroid diagnosis needs the full panel interpreted in clinical context, sometimes with antibodies and imaging. This is an educational note about where your numbers sit.
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Educational clinical estimate only — not a diagnosis, medical advice, or treatment. Scores are screening aids that must be interpreted by a qualified clinician alongside the full clinical picture. Never make care decisions from this tool alone.