FT3 to FT4 Ratio Calculator
FT3 to FT4 Ratio Calculator
Estimate peripheral T4-to-T3 conversion from free T3 and free T4 — a marker with no validated reference interval.
FT3 to FT4 Ratio
FT3 ÷ FT4Free T3 4.6 pmol/L, free T4 15.5 pmol/L
Formula
- free T3
- pmol/L
- free T4
- pmol/L
- interpretation
- reflects deiodinase-driven peripheral conversion of T4 to the more active T3, not thyroid output on its own
Worked example
Free T3 4.6 pmol/L, free T4 15.5 pmol/L
4.6 ÷ 15.5 = 0.297
Between 0.24 and 0.34 → typical range
What shifts the ratio
| Direction | Cause |
|---|---|
| Lowers the ratio | Non-thyroidal illness syndrome, amiodarone, glucocorticoids, propranolol, propylthiouracil, selenium deficiency |
| Raises the ratio | Iodine deficiency, Graves disease (preferential T3 secretion) |
| No reliable change | Levothyroxine monotherapy at a stable dose in a healthy person |
What the ratio reflects, and its real limits
Most circulating T3 is not secreted by the thyroid directly but produced peripherally by deiodinase enzymes acting on T4, chiefly deiodinase type 1 in the liver and kidney. The FT3:FT4 ratio is a crude window onto that conversion step. A low ratio is characteristic of non-thyroidal illness syndrome — sometimes called euthyroid sick syndrome — in which deiodinase type 1 activity is suppressed and reverse T3 rises instead of active T3; the same pattern of reduced conversion is seen with amiodarone, propranolol, glucocorticoids, propylthiouracil and in selenium deficiency, since selenium is a cofactor for the deiodinase enzymes. A high ratio appears in iodine deficiency, where the gland shifts synthesis toward T3 as an adaptation, and in Graves disease, where the hyperactive gland preferentially secretes T3.
Here candour matters. The ratio has become popular in functional-medicine circles as a marker of "poor conversion", used to justify starting or escalating T3-containing therapy in patients with normal TSH and normal free T4. This should be said plainly and without dismissiveness: the ratio has no validated reference interval comparable to TSH, its result depends heavily on which immunoassay platform measured the free hormones, and no trial has shown that treating a low ratio in isolation improves outcomes. It is a physiological signal worth understanding, not a diagnostic threshold, and it should not be used on its own to justify T3 therapy.
Where it is genuinely useful is as one piece of a wider picture — alongside TSH, free T4, reverse T3 where available, and the clinical context of acute illness, drug therapy or possible iodine deficiency — rather than as a number to treat toward.
Frequently asked questions
What does a low FT3:FT4 ratio mean?
Usually reduced peripheral conversion of T4 to T3. It is characteristic of non-thyroidal illness syndrome and is also seen with amiodarone, glucocorticoids, propranolol, propylthiouracil and selenium deficiency.
Is there a validated normal range for the ratio?
No. Unlike TSH, the FT3:FT4 ratio has no formally validated reference interval, and results are assay-dependent. Treat the bands here as indicative, not diagnostic.
Should a low ratio be treated with liothyronine?
Not on its own. The ratio is popular as a marker of "poor conversion" but has not been shown in trials to identify patients who benefit from T3 therapy, and major guidelines do not support using it that way.
What raises the FT3:FT4 ratio?
Iodine deficiency, where the thyroid shifts toward preferential T3 synthesis, and Graves disease, where the hyperactive gland secretes relatively more T3 than T4.
Related calculators
References
- Fliers E, Bianco AC, Langouche L, Boelen A. Thyroid function in critically ill patients. Lancet Diabetes Endocrinol. 2015;3(10):816–25.
- Jonklaas J et al. Guidelines for the treatment of hypothyroidism. American Thyroid Association Task Force. Thyroid. 2014;24(12):1670–751.