Thyroid hormone sensitivity is not measured directly — it is calculated from three values in one venous blood sample (TSH, free T4 and free T3), so it needs a lab requisition, not a home device. The indices ask a different question from a standard panel: not how much hormone is circulating, but how strongly you respond to it. No laboratory prints them; they are computed from numbers you already have.
What you need
One morning blood draw, with all three hormones measured in the same sample by the same laboratory. Ask for:
- TSH — the pituitary’s output signal, and the anchor of every central index.
- Free T4 (free thyroxine) — the hormone the pituitary is responding to.
- Free T3 (free triiodothyronine) — needed for the peripheral part.
- The units and the reference interval your lab prints — pmol/L and ng/dL are not interchangeable, the formulas are written for particular units, and the quantile-based indices score you against a reference distribution.
Context decides whether the numbers are usable. Book repeats into the same morning slot, since TSH is pulsatile and follows a circadian rhythm. Do not test during an acute illness or while fasting hard, and pause high-dose biotin. Pregnancy, estrogen, amiodarone and lithium all shift the inputs. On levothyroxine or an antithyroid drug the loop is driven from outside, and the indices lose most of their meaning.
How the measurement works
The pituitary reads circulating free T4 and sets TSH against it, and the relationship is negative and roughly log-linear: as free T4 rises, TSH falls steeply. Sensitivity indices exploit that, comparing the TSH you have with the TSH your free T4 should have produced. A TSH higher than that pairing predicts means the pituitary is suppressed less than expected — reduced central sensitivity.
Jostel’s TSH index (TSHI) combines the logarithm of TSH with free T4, rising when TSH is high for the accompanying free T4. The parametric thyroid feedback quantile-based index (PTFQI) converts each value into its position within a reference population and subtracts one from the other; it spans −1 to +1, centers near zero, and points the same way as TSHI. The free T3 to free T4 ratio moves the question out of the brain: how much precursor your tissues convert into the active hormone. Computing all of them from one set of values, in your own units, is what Thyroid hormone sensitivity does; if conversion is the part you care about, a Peripheral thyroid hormone conversion assessment takes that step alone.
How to read the result
Read direction before magnitude. These indices have no consensus clinical thresholds, so the table gives direction, not a diagnosis.
| Index | What it compares | What a higher value suggests |
|---|---|---|
| TSHI (Jostel) | TSH against free T4 in the same sample | The pituitary is less suppressed than that free T4 predicts — lower central sensitivity |
| PTFQI | Where free T4 and TSH sit in a reference distribution | The same direction as TSHI, on a scale centered near zero |
| Free T3 / free T4 | Active hormone against its precursor | More peripheral conversion; a low ratio points to less |
Any band a calculator shows comes from a reference population on one assay, and free T4 immunoassays disagree enough that two labs are not comparable. Treat a single result as a position, not a verdict, and weight the direction across repeats in the same laboratory. In cohorts, reduced central sensitivity travels with obesity and diabetes, but that is an association across populations.
What it does not tell you
It does not tell you whether your thyroid is working. That is the panel’s job: a high index is not hypothyroidism, and someone with a frankly abnormal TSH can return an unremarkable score. Nor does it say why: receptor-mutation resistance is rare, and a raised index far more often accompanies obesity or insulin resistance, with the data unable to say which came first.
It is also fragile. One draw catches one moment of a pulsatile signal, and non-thyroidal illness, energy restriction, biotin and interfering antibodies all move the inputs without changing sensitivity. Validation in pregnancy, in children and in treated patients is thin.
Finally, it describes a relationship, not a capacity and not an effect. It says nothing about how much hormone the gland could still produce — Maximum thyroid secretory capacity is the parameter for that — nor about what hormone does at the receptor inside a cell, which no blood test reaches. No trial has shown that treating to move one of these numbers improves anything.
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.