Thyroid hormone sensitivity
Hormone levels are only half the story. The other half is how sharply the pituitary and the tissues respond to them. That “tuning” is captured by sensitivity indices: the central TSHI and PTFQI describe the feedback of the pituitary ↔ thyroid axis, while the free T3/free T4 ratio reflects the response of peripheral tissues. The calculator computes them from three blood tests and catches subtle shifts a TSH alone would miss.
What thyroid hormone sensitivity is
The thyroid gland's work is governed by feedback with the pituitary: it releases TSH, stimulating the production of thyroxine (T4) and triiodothyronine (T3). Free T4 and T3, in turn, suppress the release of TSH — these are two negative-feedback loops. Sensitivity is the “steepness” of that response: in some people the pituitary reacts to a change in hormones sharply, in others sluggishly. It is precisely the shift in sensitivity, not just the absolute hormone level, that is linked to metabolic syndrome, diabetes and other risks.
Thyroid hormone sensitivity calculator
Enter three values from your blood test — TSH, free T4 and free T3. The calculation runs on the server; your data is not stored.
How to read the sensitivity indices
| Index | What it means | Normal range |
|---|---|---|
| TSHI | Central sensitivity: how “sharply” the pituitary responds to free T4. The primary, robust index | 1.35–3.71 |
| PTFQI | A rank-based thyroxine feedback index — compares where free T4 and TSH sit. Supporting (see below) | −0.4…0.4 |
| free T3 / free T4 | Peripheral sensitivity: how well the reserve T4 is converted into active T3 in the tissues | 0.29–0.38 |
High TSHI and PTFQI point to reduced central sensitivity (TSH higher than expected for a given free T4 — the picture of subclinical hypothyroidism), low values to increased sensitivity (TSH suppressed relative to free T4). For the free T3/free T4 ratio, low values indicate slowed peripheral conversion, high values enhanced conversion.
What shifts hormone sensitivity
- Metabolic background — obesity, insulin resistance and metabolic syndrome are linked to reduced sensitivity to thyroid hormones.
- Chronic stress and high cortisol — alter both the central tuning and the peripheral conversion. A stress level test can help gauge the background.
- Nutrient deficiencies — selenium, zinc, iron: the conversion of T4 into active T3 suffers and the free T3/free T4 ratio falls.
- Autoimmune processes — early Hashimoto's or Graves' change the hormone ratio even before overt shifts in TSH.
- Thyroid medication — T4 monotherapy or T3-containing agents shift the indices; interpret them in light of the treatment.
When it's worth assessing sensitivity
If your TSH is technically “normal” but fatigue, weight gain, feeling cold and “brain fog” persist, a shift in sensitivity helps explain symptoms where the individual hormones look normal. If you don't have lab tests yet, start with a symptom-based screening, and come back here once you have the results.
Frequently asked questions
How is sensitivity different from hormone levels?
Hormone levels show how much of them is in the blood. Sensitivity is how strongly the system responds to them: how the pituitary changes TSH in response to free T4 (centrally) and how the tissues convert T4 into active T3 (peripherally). Two people with identical hormone levels can have different sensitivity.
Why does PTFQI show “reduced sensitivity” even though I'm healthy?
PTFQI is calibrated against the NHANES population using a particular free T4 immunoassay. Many modern laboratories measure free T4 on a higher scale, which systematically inflates the index in healthy people. That is why we show it as a supporting measure, with a caveat, and exclude it from the overall verdict — rely first on TSHI and on the trend over time.
Which lab tests are needed for the calculation?
Three values from a blood test: TSH, free T4 and free T3. Their basic interpretation against reference ranges is provided by the thyroid function assessment.
How is this different from other thyroid calculators?
The focus here is on sensitivity: the tuning of the feedback (TSHI, PTFQI) and the tissue response (free T3/free T4). The conversion assessment examines the T4→T3 step in more detail (SPINA-GD), while the comprehensive assessment adds antibodies and secretory capacity.
