Peripheral thyroid hormone conversion assessment
The thyroid gland mainly produces the prohormone T4, while the active hormone is T3. In a healthy person, about 80% of T3 is formed not in the gland but in the tissues — by converting T4 under the action of deiodinase enzymes. If this conversion is impaired, the symptoms of hypothyroidism persist even with a normal TSH. The calculator assesses conversion via SPINA-GD and the free T3/free T4 ratio.
Why T4-to-T3 conversion matters so much
T4 is a “reserve” that is barely active on its own. The real work in the cells is done by T3: it binds to receptors in the cell nuclei and regulates the basal metabolism. Only a small share of T3 is released by the gland itself — most is formed in the periphery, when deiodinase enzymes “strip” iodine from T4. If this process is slowed (stress, selenium/zinc deficiency, inflammation, illness), a person can have a normal TSH and even a normal free T4, but a low free T3 — along with persistent symptoms of hypothyroidism.
T4→T3 conversion calculator
Enter three values from your blood test — TSH, free T4 and free T3. The calculation runs on the server and the data is not stored.
How to read SPINA-GD and free T3/free T4
| Marker | What it means | Normal |
|---|---|---|
| SPINA-GD | The total activity of the deiodinases — the “power” of T4-to-T3 conversion | 20–40 nmol/s |
| free T3 / free T4 | The ratio of the active to the reserve hormone: conversion quality | 0.29–0.38 |
Low values point to slowed conversion (a shortage of active T3), high ones to increased conversion (early Hashimoto's/Graves', taking T3-containing medication, metabolic states).
What impairs T4-to-T3 conversion
- Nutrient deficiencies — of selenium, zinc and iron, which the deiodinases need to work.
- Chronic stress and high cortisol — slow conversion and increase the formation of inactive reverse T3. The PSS-10 test helps gauge stress.
- Inflammation and illness — acute and chronic conditions reduce conversion (low-T3 syndrome).
- Crash diets and a calorie deficit — the body saves energy by slowing the metabolism.
- T4-only monotherapy (levothyroxine) — with poor conversion, free T3 stays low even with a normal TSH.
When it is worth assessing conversion
If TSH is “normal” yet fatigue, weight gain, feeling cold, brain “fog” and hair loss persist, it is worth looking specifically at free T3 and conversion. If you do not have lab results yet, start with the symptom-based screening, and come back here once you have the results in hand.
Frequently asked questions
My TSH and free T4 are normal, but I feel unwell. Could it be a conversion problem?
Yes, this is the classic picture of impaired conversion: there is enough T4, but it converts poorly into active T3. That is exactly why it is important to look at free T3, SPINA-GD and the free T3/free T4 ratio, not just TSH.
Which lab tests are needed for the calculation?
Three values from a blood test: TSH, free T4 and free T3. That's all — nothing else needs to be entered. A basic interpretation of them against the reference ranges is given by the thyroid function assessment.
How is this different from the comprehensive thyroid assessment?
This is only about T4→T3 conversion (SPINA-GD and free T3/free T4). The comprehensive assessment additionally looks at antibodies, central sensitivity (TSHI) and secretory capacity (SPINA-GT).
What should I do about low conversion?
Discuss the result with an endocrinologist. Correcting deficiencies (selenium, zinc, iron), managing stress and sleep, and eating enough calories often help; in some cases a doctor will consider T3-based therapy. Self-medication is not acceptable.
