Thyroid Sensitivity Index (TSHI)

What is the TSH Index?

The Thyrotroph T4 Sensitivity Index (TSHI) is a calculated value that helps quantify the sensitivity of the pituitary gland to thyroid hormone negative feedback. It integrates both TSH and free T4 levels into a single, standardized score.

This index is particularly useful for identifying subtle abnormalities in the hypothalamic-pituitary-thyroid (HPT) axis that may not be apparent from looking at TSH or FT4 values alone.

Why is the TSHI Clinically Relevant?

• Helps in diagnosing central hypothyroidism, where TSH may be inappropriately "normal" despite low FT4.
• Can identify states of thyroid hormone resistance, where TSH is elevated despite normal or high FT4 levels.
• Provides a standardized method to assess the pituitary-thyroid feedback loop in research and complex clinical cases.

Interpreting the Result (Reference Guide)

Important Clinical Considerations

This index does not have universally standardized reference ranges and is highly dependent on the specific assays used. It should always be interpreted as part of a broader clinical evaluation.

• A significantly high index is a strong indicator for RTH and warrants further investigation.
• A very low index can be seen in central hypothyroidism.
• Values can be affected by non-thyroidal illness ("euthyroid sick syndrome").
• Genetic testing is the gold standard for confirming Resistance to Thyroid Hormone.