Calculator

Thyroid Conversion Calculator

Put your TSH, free T4, and free T3 in context: position within reference ranges, the FT3:FT4 conversion ratio, and what the combined pattern suggests. Educational context only, not a diagnosis.

Reference ranges auto-fill with typical values. Edit them to match the ranges printed on your lab report; they vary by assay.

Leave blank if not on your report. Reverse T3 enables the disputed FT3:rT3 ratio; total T4/T3 help flag binding-protein effects (estrogen, pregnancy, illness).

How this works

The thyroid gland secretes mostly T4, a prohormone. Roughly 80% of the active hormone T3 in your blood is made outside the thyroid, when deiodinase enzymes (D1 and D2) strip an outer-ring iodine from T4; only about 20% is secreted directly. A third enzyme, D3, inactivates T4 into reverse T3 and T3 into T2. TSH is set mainly by the hypothalamus and pituitary sensing T4 (converted locally by pituitary D2), which is why serum T3 can drift low while TSH stays normal: the pituitary is watching a different signal than your peripheral tissues receive.

The FT3:FT4 ratio is an accepted research proxy for peripheral deiodinase activity. In molar units (both hormones in pmol/L), euthyroid population means cluster around 0.31, and most adults fall roughly between 0.20 and 0.45; this calculator converts your inputs to pmol/L (FT4 ng/dL × 12.87, FT3 pg/mL × 1.536) and uses those bands. Lower ratios have been associated with worse outcomes in cohort studies (in NHANES 2007-2012, each standard deviation of higher FT3/FT4 predicted a hazard ratio of 0.77 for all-cause mortality), but association is not diagnosis: no guideline defines a disease threshold for this ratio, and it is not a validated diagnostic test.

Reverse T3 and the FT3:rT3 ratio are more contested. The informal cutoff circulated online (FT3 in pg/mL divided by rT3 in ng/dL, "healthy above 0.2") compares a free hormone with a total hormone and has no outcome data behind it. Mainstream endocrinology, including the ATA treatment guidelines, does not recommend rT3 testing for diagnosing or managing hypothyroidism. This page reports it only to explain what your result does and does not mean.

Conversion is physiologically suppressible. Sustained calorie restriction lowers serum T3 within days (an adaptive energy-sparing response that reverses with refeeding). All three deiodinases are selenoproteins, so frank selenium deficiency impairs them; zinc deficiency has similar though weaker evidence. Systemic illness and inflammatory cytokines shift conversion toward rT3 (non-thyroidal illness syndrome), and glucocorticoid excess, amiodarone, propylthiouracil, and high-dose propranolol inhibit D1.

This tool does not diagnose anything. It contextualizes numbers. Reference ranges are assay-specific, single measurements fluctuate, and biotin supplements can corrupt thyroid immunoassays (falsely high FT4/FT3, falsely low TSH on some platforms). Bring questions to the clinician who ordered the test; do not start, stop, or adjust thyroid medication based on this page.

References

Bianco AC, et al. Paradigms of Dynamic Control of Thyroid Hormone Signaling. Endocr Rev. 2019;40(4):1000-1047.

Fliers E, Boelen A. An update on non-thyroidal illness syndrome. J Endocrinol Invest. 2021;44(8):1597-1607.

Lang X, et al. FT3/FT4 ratio is correlated with all-cause mortality, cardiovascular mortality, and cardiovascular disease risk: NHANES 2007-2012. Front Endocrinol (Lausanne). 2022;13:964822.

Jonklaas J, et al. Guidelines for the Treatment of Hypothyroidism: Prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement. Thyroid. 2014;24(12):1670-1751.