Peripheral conversion is the share of the storage hormone T4 that your tissues turn into active T3, and it is not measured directly — it is calculated from free T4 and free T3 in one venous blood sample, so it needs a lab requisition, not a home device. No test reads the deiodinase enzymes themselves. A calculator takes the two hormone concentrations at steady state and works backwards to the enzyme activity that would explain them; the best known version of that estimate is SPINA-GD, the summed activity of the peripheral deiodinases.
What you need
- Free T4 and free T3 from the same sample, run by the same laboratory. The estimate compares the two, so values from different days or different labs cannot be combined.
- TSH from that draw. Not part of the calculation, but it tells you whether the gland and pituitary are in a state where the estimate means anything.
- The units your report prints — pmol/L, ng/dL, pg/mL. The constants are unit-specific, and a mis-entered unit returns a confident, wrong number.
- A steady state: no levothyroxine dose change for about six weeks, and no acute illness, surgery or crash diet beforehand.
- Your drug list. Amiodarone, propylthiouracil, glucocorticoids and high-dose propranolol suppress deiodination; high-dose biotin distorts the assay itself.
Read the panel itself first: a conversion estimate only makes sense once you know whether you are euthyroid, treated, or frankly under- or overactive — the question Thyroid function assessment answers from the same three values.
How the measurement works
The thyroid secretes mostly T4, which is close to inert. Most T3 in your blood was made elsewhere: deiodinase enzymes, chiefly in liver and kidney, strip an iodine atom from T4 and release active hormone into the circulation. A parallel enzyme inactivates both hormones, producing reverse T3 among others, so what circulates is the net of the two.
Since those enzymes cannot be sampled, the estimate comes from the equilibrium instead. A structure-parameter model treats free T4 as the substrate and free T3 as the product, corrects for protein binding and enzyme saturation, and returns the converting activity that would sustain that pair of concentrations, as a rate in nanomoles per second. The direction of travel is what you read: for a given free T4, a higher free T3 raises the estimate; for a given free T3, a higher free T4 lowers it. It is not a plain ratio and not worth attempting by hand — the published constants are what Peripheral thyroid hormone conversion assessment carries, so you enter the two values with their units and let it do the arithmetic.
How to read the result
Read it as a position within the range your calculator states, and as a pattern with the hormones that produced it.
| Pattern in the blood | Effect on the estimate | What usually explains it |
|---|---|---|
| Free T3 low or low-normal, free T4 mid-range or higher | Pushes it down | Illness, fasting, drugs that block deiodination, levothyroxine monotherapy in some people |
| Free T4 and free T3 proportionate, both mid-range | Lands mid-range | The ordinary euthyroid pattern |
| Free T3 high relative to free T4 | Pushes it up | Graves’ disease and autonomous nodules, which secrete relatively more T3; iodine deficiency; any therapy containing T3 |
The reference interval commonly quoted for SPINA-GD is roughly 20 to 40 nmol/s, but it comes from one population measured on particular assays, so the range printed beside your own result is the one that applies. Treat a value just outside a border as noise until a repeat draw, under the same conditions, says otherwise.
What it does not tell you
It is a model output, not a measurement. Every error in the two immunoassays passes into it, and free-hormone assays disagree between analyzer platforms; pregnancy, heparin, biotin and interfering antibodies shift the inputs. The model assumes a steady state, so in serious illness — when T3 falls as a normal adaptation — a low estimate describes the illness, not the thyroid.
Blood is also blind to the tissue that matters most. A second deiodinase works inside the brain, pituitary, brown fat and muscle, making T3 locally for those cells; that hormone largely never enters the circulation, so no blood-derived estimate can say what your brain is receiving. It says nothing about the gland’s own output reserve — a separate parameter, the subject of Maximum thyroid secretory capacity — and nothing about autoimmunity, nodules or cancer.
Above all it is neither a diagnosis nor a prescription. Poor conversion is not a recognized disease entity, a low value does not establish that adding T3 to treatment would help, and trials of combined T4 plus T3 therapy have given mixed results against T4 alone. No outcome trial shows that treating this number makes people live longer or better, and symptoms like fatigue and low mood are too non-specific to be settled by it. Where the pattern points toward hypothyroidism, the decision belongs with a clinician reading the whole panel.
Related reading
Disclaimer. This article is for information only and does not replace medical advice. Talk to a qualified clinician before changing anything about your health.