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Biomarkers

TSHI — Jostel’s TSH Index

The adequacy of TSH to the free-T4 level

The pituitary–thyroid feedback loop and the calculated TSHI index

TSHI (Jostel’s TSH Index) is a calculated measure of the pituitary’s thyrotropic function. It reflects the fact that TSH should not be judged on its own but together with free T4, since the two are linked by feedback.

The index helps detect central (secondary) hypothyroidism and assess sensitivity to thyroid hormones. It is not a separate blood test but a formula based on TSH and free T4 that have already been measured.

Reference ranges

Formula and reference values (free T4 is entered in pmol/L):

FormulaTSHI = ln(TSH) + 0.1345 × FT4
Reference1.3 – 4.1
Standardized sTSHI(TSHI − 2.7) / 0.676
Important: Units matter: free T4 in pmol/L, TSH in mIU/L. The index is computed by software or a calculator.

What an elevated level means

A high TSHI points to reduced tissue sensitivity to thyroid hormones: the pituitary keeps TSH high while free T4 is normal. In studies this state is linked to obesity, subclinical hypothyroidism and metabolic disturbances.

What a low level means

A low TSHI reflects underactivity of the pituitary — it is used to detect central (secondary) hypothyroidism, when TSH is “inappropriately low” despite reduced free T4. European guidelines on central hypothyroidism call the index a useful additional tool.

How the test is done

There is no separate blood test for TSHI: it needs TSH and free T4 measured at the same time from one sample. The value is derived from the formula; related indices (PTFQI, SPINA-GT) assess other links in the regulation.

What affects the value

  • Units of free T4 (pmol/L are required) — entering ng/dL gives a wrong result.
  • Taking thyroxine and an unstable thyroid function distort interpretation.
  • The reference depends on the population and on the assay used to measure the hormones.

Calculate using this marker

Calculators where TSHI (Thyroid Sensitivity Index) is used directly:

Thyroid Integral Index Assessing central and peripheral sensitivity to thyroid hormones Open Thyroid Function TSH, free T4 and T3 panel: hormone status and the likely diagnosis Open

Frequently asked questions

How is TSHI different from ordinary TSH?

TSH is a single value, whereas TSHI combines TSH and free T4 into one number that reflects pituitary function and sensitivity to hormones. It is more informative in borderline and “illogical” combinations of results.

Where can I calculate TSHI?

With the formula or a dedicated calculator into which you enter TSH and free T4 measured at the same time. It is important to use the correct units (free T4 in pmol/L).

Related terms

TSH (Thyroid-stimulating hormone) The pituitary hormone that controls the thyroid gland Free thyroxine (T4) The main thyroid hormone PTFQI (Parametric Thyroid Feedback Quantile-based Index) Rank-based sensitivity of the pituitary to free T4 SPINA-GT (secretory capacity) The gland's maximum thyroxine output SPINA-GD (deiodinase activity) Peripheral conversion of T4 → T3 Thyroid gland The endocrine organ that governs metabolism Subclinical disorders Early shifts in function without overt symptoms

Sources

  1. Jostel’s TSH index — Wikipedia
  2. Jostel A. et al. Definition and evaluation of the TSH Index. Clin Endocrinol, 2009 (PMID 19226261)
info
This content is educational and does not replace consultation with a physician. Reference ranges may vary depending on the laboratory and method — rely on the ranges stated on your own test report.
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