Thyroid Conversion Calculator: How the FT3/FT4 Ratio Works, Step by Step

A thyroid conversion calculator does one small piece of maths: it divides your free T3 (FT3) by your free T4 (FT4) to give a ratio that hints at how much of the storage hormone T4 your body is turning into the active hormone T3. In the units most United States labs print, the formula is FT3 (pg/mL) ÷ FT4 (ng/dL). That is the whole calculation — and it is also where almost everyone goes wrong, because a second, equally legitimate version of the same ratio is calculated in molar units (pmol/L) and produces a number about ten times smaller. This page teaches the calculation itself: how T4-to-T3 conversion works in the body, both formulas with their exact unit conversions, three fully worked hypothetical examples with every step shown, how to read the units on your own lab report, and — just as important — what this ratio cannot tell you. There is no interactive calculator here on purpose: once you understand the units, you can do this reliably with any pocket calculator, and you will never be fooled by two websites giving you wildly different “answers” from the same blood test.

How T4-to-T3 conversion actually works

Your thyroid gland releases mostly T4 (thyroxine) and a smaller amount of T3 (triiodothyronine) — roughly 80% T4 and 20% T3. T4 is best thought of as a prohormone or storage form: it circulates for about a week, but it has little activity of its own. T3 is the form that binds thyroid hormone receptors and drives metabolic activity, and most of it is not secreted directly. Reviews of thyroid physiology estimate that around 80% of the body’s T3 is produced outside the thyroid by peripheral conversion of T4, with only about 20% coming straight from the gland (Mullur et al., Physiological Reviews, via PMC ).

Conversion is done by a family of enzymes called iodothyronine deiodinases. All three are selenoproteins — they contain selenium — which is one reason selenium status shows up in the conversion literature. Type 1 deiodinase (D1), found mainly in the liver, kidneys and thyroid, and type 2 deiodinase (D2), found in the brain, pituitary, skeletal muscle, brown adipose tissue and thyroid, remove one iodine atom from the outer ring of T4 and produce active T3. D2 is especially important for local, tissue-level T3 production — for example, much of the T3 acting in the brain and pituitary is made on site by D2. Type 3 deiodinase (D3) does the opposite job: it removes an iodine atom from the inner ring, converting T4 into inactive reverse T3 (rT3) and breaking T3 down into inactive T2. Your tissues constantly tune the balance between D2 and D3 activity, which is how the same blood level of T4 can produce different amounts of T3 signalling in different organs and different situations (Bianco & Kim, Journal of Clinical Investigation, via PMC ; Bianco et al., Endocrine Reviews, via PMC ).

Two practical consequences follow. First, a blood FT3/FT4 ratio is at best a rough, whole-body shadow of conversion — it cannot see what D2 is doing inside your brain or muscles. Second, anything that shifts deiodinase activity — illness, inflammation, fasting, certain medicines — can move the ratio without anything being wrong with the thyroid gland itself. Both points matter when you interpret a number later.

One piece of chemistry worth knowing: T4 and T3 are built on the amino acid tyrosine — the gland attaches three or four iodine atoms to tyrosine residues, which is also why the hormones are named tri-iodothyronine and tetra-iodothyronine. For the amino acid itself — its roles in neurotransmitter production, dosage and safety as a supplement — see our separate guide to tyrosine. It is a different topic from the lab calculation on this page.

The FT3/FT4 ratio: the formula, stated exactly

The ratio only makes sense under four conditions, and every source that handles it carefully states them: both values must be free hormone (FT3 and FT4 — never mix a total T3 with a free T4), from the same blood draw, ideally measured by the same laboratory on the same assay platform, and divided in a single, stated unit system. There are two unit systems in routine use, and they give different numbers for the same blood.

System 1 — conventional (US) units, the bedside ratio. FT3 is reported in pg/mL and FT4 in ng/dL, and the ratio is a direct division of the two printed numbers:

US FT3/FT4 ratio = FT3 (pg/mL) ÷ FT4 (ng/dL)

System 2 — molar (SI) units, the research ratio. Both hormones are converted to pmol/L first, and then divided. The conversion factors are fixed:

FT3 in pmol/L = FT3 in pg/mL × 1.536
FT4 in pmol/L = FT4 in ng/dL × 12.87
Molar FT3/FT4 ratio = FT3 (pmol/L) ÷ FT4 (pmol/L)

To go the other way: FT3 in pg/mL = FT3 in pmol/L ÷ 1.536 (equivalently × 0.651), and FT4 in ng/dL = FT4 in pmol/L ÷ 12.87. Because the two factors differ, the two ratios are linked by a bridge factor: molar ratio ≈ US ratio × 0.119 (that is 1.536 ÷ 12.87). So a US ratio of 2.91 and a molar ratio of 0.35 can be the exact same blood sample. Neither is wrong — but comparing a number from one system against a reference range from the other is the single most common error in this whole topic, and several popular calculator pages make it worse by printing a third variant (dividing FT3 by FT4 × 10 to align the mass units, which for typical values gives ≈0.29) next to reference ranges that belong to the first system. Before you compare your ratio to anything, write down which system produced it.

How to calculate your ratio in five steps

1. Find FT3 and FT4 on your report — the lines labelled “Free T3” / “Triiodothyronine, Free” and “Free T4” / “Thyroxine, Free”. Ignore the “Total T3” and “Total T4” lines for this calculation.
2. Copy the unit printed next to each result. In the US that is usually pg/mL for FT3 and ng/dL for FT4; many other countries print pmol/L for both.
3. Pick one system and convert if needed using the factors above. If both results are already in pmol/L, divide directly for the molar ratio. If FT3 is in pg/mL and FT4 in ng/dL, divide directly for the US ratio — do not convert one and not the other.
4. Divide FT3 by FT4 and round to two decimal places (US system) or two to three decimal places (molar system).
5. Label the result with its system — for example “2.91 (US units)” or “0.35 (molar)” — and compare it only with ranges published in that same system, alongside the absolute FT3, FT4 and TSH values and the reference ranges printed on your own report.

Worked examples (hypothetical — for learning the maths)

The three examples below use invented lab values chosen to sit inside typical adult reference ranges. They are teaching examples, not anyone’s real results, and every step was checked twice — including by converting each answer back through the bridge factor (US ratio × 0.119 ≈ molar ratio).

Example 1: a US-units report, mid-range values

Hypothetical example. FT3 = 3.2 pg/mL, FT4 = 1.1 ng/dL.

• US ratio: 3.2 ÷ 1.1 = 2.91.
• Convert to molar units: FT3 = 3.2 × 1.536 = 4.9152 pmol/L. FT4 = 1.1 × 12.87 = 14.157 pmol/L.
• Molar ratio: 4.9152 ÷ 14.157 = 0.35 (0.347 before rounding).
• Check: 2.91 × 0.119 ≈ 0.35. ✓ Consistent. The mass-aligned variant some calculators use, 3.2 ÷ (1.1 × 10) = 0.29, is a third number for the same blood — which is why an unlabelled “0.29” and an unlabelled “2.91” online may not disagree at all.

Example 2: an SI-units report, divided directly

Hypothetical example. FT3 = 4.6 pmol/L, FT4 = 15.0 pmol/L.

• Molar ratio: 4.6 ÷ 15.0 = 0.31.
• Convert to US units to cross-check: FT3 = 4.6 ÷ 1.536 = 2.99 pg/mL. FT4 = 15.0 ÷ 12.87 = 1.17 ng/dL.
• US ratio: 2.99 ÷ 1.17 = 2.57. Check: 2.57 × 0.119 ≈ 0.31. ✓ Consistent.

Example 3: a lower-ratio pattern

Hypothetical example. FT3 = 2.4 pg/mL, FT4 = 1.4 ng/dL — FT3 near the bottom of its typical range with FT4 in the upper half, a pattern clinicians sometimes describe as relatively less T3 per unit of T4.

• US ratio: 2.4 ÷ 1.4 = 1.71.
• Molar check: FT3 = 2.4 × 1.536 = 3.6864 pmol/L. FT4 = 1.4 × 12.87 = 18.018 pmol/L. Molar ratio: 3.6864 ÷ 18.018 = 0.20. Check: 1.71 × 0.119 ≈ 0.20. ✓ Consistent.
• What it does and does not mean: the arithmetic is solid, but the pattern alone does not diagnose “poor conversion”. The same picture can appear during or after illness, with under-eating, with certain medicines, or simply on levothyroxine treatment (see below) — and the absolute values, TSH, symptoms and history decide what, if anything, it signifies. That judgement belongs with your clinician.

The pg/dL trap. A few labs report FT3 in pg/dL instead of pg/mL. Since 1 pg/mL = 100 pg/dL, an FT3 of 320 pg/dL is the same result as 3.2 pg/mL — divide by 100 before doing anything else. If you divided 320 by an FT4 of 1.1 ng/dL without converting, you would get 290.9, a meaningless number one hundred times too large. If a ratio ever comes out in the hundreds, a unit has been misread.

How to read your own lab report’s units

Work down the report in this order. First, confirm the tests are the free hormones: reports variously label them “Free T3”, “FT3”, “Triiodothyronine (T3), Free” and “Free T4”, “FT4”, “Thyroxine (T4), Free”. Total T3 and total T4 measure mostly protein-bound hormone and must not be substituted — there is no fixed free-to-total proportion you can correct with. Second, read the unit column literally, result by result; do not assume the whole panel uses one system, and watch especially for pg/dL (see the trap above) and for pmol/L reports from non-US labs. Third, note the reference range printed beside each result — that range, from your laboratory’s own assay, is the one that applies to your numbers, because FT3 and FT4 immunoassays differ between manufacturers and platforms and results are not perfectly interchangeable between labs (American Thyroid Association: Thyroid Function Tests). Fourth, keep the context with the numbers: the time of day, whether you were ill, fasting or recovering from illness, pregnancy, and every medicine and supplement you were taking — tell the laboratory and your clinician about biotin-containing supplements in particular, as biotin is a well-known source of interference in immunoassays. Finally, if you track a ratio over time, calculate it the same way, in the same system, ideally from the same laboratory, or the trend you see may be a change of method rather than a change in you. If thyroid symptoms are what prompted the test in the first place, our overview of hypothyroidism symptoms explains which patterns clinicians take seriously.

What counts as a “typical” ratio? The sources disagree

Honesty requires a table, not a single number, because published “typical” values were produced in different unit systems, populations and assay platforms:

• Molar system, healthy untreated adults: in the large study by Gullo and colleagues (3,875 euthyroid controls), the average FT3/FT4 ratio was about 0.31–0.33, and a widely cited summary of that work gives a control reference range of roughly 0.20–0.50 (Gullo et al., PLoS ONE, via PMC ).
• US system, laboratory-medicine sources: one functional-lab reference lists a standard range of 2.2–2.9 with a narrower “optimal” band of 2.4–2.7 (FT3 pg/mL ÷ FT4 ng/dL), while several patient-facing calculators describe roughly 2.0–3.0, and one clinician’s practice describes people who feel well as “around 3.0 or higher” with below about 2.5 “looking like a conversion problem”. These are practice conventions, not validated cut-offs.
• US system, population cohorts: reported means alone span about 2.3 to 2.9 depending on sex and population (for example 2.5 in men and 2.3 in women in one European cohort, versus 2.88 and 2.75 in another) — before any question of a cut-off is even reached.

Read those three bullets together and the conclusion writes itself: there is no single validated “normal FT3/FT4 ratio” you can grade yourself against. The molar and US figures describe the same physiology on different scales (0.32 molar ≈ 2.7 US, using the ×0.119 bridge), and within each scale, healthy averages move with age, sex, population and assay. A ratio modestly below a website’s favourite number is not, by itself, evidence of anything.

Factors that can lower conversion or the ratio

The literature links a lower FT3 relative to FT4 — and in many cases a diversion of T4 towards reverse T3 — to the following. Treat this as background for a conversation with your clinician, not as a self-diagnosis checklist:

• Illness and inflammation. Acute and chronic illness, including critical illness, infection, inflammation, surgery and heart failure, characteristically suppress D1/D2 activity and raise D3 activity, lowering T3 while reverse T3 climbs — the pattern often called non-thyroidal illness or “low T3 syndrome”. It is frequently an adaptive response to illness rather than a thyroid defect, and it is a major reason a ratio measured while you are unwell should not be over-interpreted.
• Under-eating. Fasting, starvation and severe calorie restriction reduce peripheral conversion and raise reverse T3 in classic physiology studies; the effect reverses with refeeding.
• Selenium deficiency. Because all three deiodinases are selenoproteins, selenium deficiency can impair deiodinase activity in experimental and clinical literature. That supports correcting a genuine deficiency — it does not support high-dose selenium without testing and advice, and selenium excess is itself harmful. Our guide to thyroid vitamins covers where nutrients genuinely fit.
• Certain medicines. High-dose propranolol and other beta-blocker regimens, glucocorticoids, amiodarone and propylthiouracil have all been reported to reduce peripheral T4-to-T3 conversion. Never stop or change a prescribed medicine because of a ratio — raise it with the prescriber instead.
• Age and physiology. Population studies find the average ratio drifts down with age and differs slightly by sex, and it shifts in pregnancy — another reason a single population number cannot grade an individual.
• Liver and kidney disease can alter deiodinase activity and binding proteins, distorting both the hormone levels and any ratio built from them.

The medication context: levothyroxine and T4/T3 combinations

Most people calculating this ratio are being treated for hypothyroidism, so one well-supported finding belongs here. Levothyroxine (T4-only) is the standard treatment for hypothyroidism (NIDDK: Hypothyroidism ), and it relies entirely on the patient’s own conversion to supply T3. In Gullo and colleagues’ study, 1,811 patients without a thyroid gland who were taking levothyroxine with a normal TSH had, on average, higher FT4, lower FT3 and a lower FT3/FT4 ratio than healthy controls — about 0.23–0.24 molar versus 0.31–0.33 — and 15.2% of them had an FT3 below the control reference range despite the normal TSH, with wide individual variation in T3 production capacity (Gullo et al., via PMC ). In other words, a lower ratio on levothyroxine is a recognised, common pattern — not proof that your dose is wrong, and not, on its own, a reason to change it.

Combination treatment with levothyroxine plus liothyronine (T3) exists, and guidelines discuss it cautiously for selected patients under specialist supervision; whether it improves outcomes, and for whom, remains debated in the literature. Separately, beware a naming collision in search results: some “T4 to T3 calculators” are not lab calculators at all — they are dose-ratio tools that split a tablet dose by proportions such as 5:1, based on the gland’s roughly 80/20 T4:T3 secretion. A dose ratio in micrograms and a lab FT3/FT4 ratio are completely different calculations that happen to share a name. Never adjust a thyroid dose — yours or anyone’s — from either kind of ratio without the prescribing clinician; our overview of hypothyroidism medication explains the treatment landscape this decision sits in.

The accuracy frame: what this ratio cannot do

Hold these limits as firmly as the formula. The FT3/FT4 ratio is not a validated stand-alone diagnostic test: no major guideline diagnoses hypothyroidism, hyperthyroidism or a “conversion disorder” from this ratio alone, and no treatment target for the ratio has been validated in trials. It must always be read anchored to the absolute values — a ratio of 0.35 built from two low-normal values and the same ratio built from two high values are different clinical situations, and the identical ratio can even appear in opposite conditions (a high ratio around 0.46, for example, has been described both in early autoimmune hypothyroidism, where a falling FT4 inflates the ratio, and in early Graves’ hyperthyroidism). Add TSH, symptoms, history and the reason the test was ordered before drawing any conclusion. Reference ranges vary by laboratory, assay, age and population — sometimes substantially — and free-hormone immunoassays are estimates that different platforms do not always agree on. Timing matters too: hormone levels move across the day, after a dose of levothyroxine, and during illness or pregnancy. The right use of the calculation on this page is narrower and genuinely useful: to understand your own report, to spot a unit error, to compare like with like over time, and to arrive at an appointment with sharper questions. Interpretation — and any change in treatment — belongs with your clinician.

The bottom line

A thyroid conversion calculator divides FT3 by FT4 — FT3 (pg/mL) ÷ FT4 (ng/dL) in US units, giving typical healthy averages around 2.3–2.9, or FT3 ÷ FT4 in pmol/L after multiplying pg/mL by 1.536 and ng/dL by 12.87, giving averages around 0.31–0.33. The two scales are linked by ×0.119, and most online confusion is one scale’s number graded against the other scale’s range. Do the division carefully, label the system, anchor the result in the absolute values and your own lab’s ranges — and let a clinician do the interpreting.

Educational disclaimer: This article teaches a calculation and explains published research for general education. It is not medical advice, does not diagnose any condition, and must not be used to start, stop or adjust thyroid medication. If your results concern you, discuss them with your own clinician, who knows your history and your laboratory’s methods.