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Thyroid Index (TSH/FT4/FT3 Interpretation)

· Last updated：2026-08-09

This tool is for informational purposes only and does not constitute medical, financial, or legal advice; consult a qualified professional for advice.

Interpret the thyroid axis by pattern-reading TSH with FT4 and (optionally) FT3. Adult reference ranges: TSH 0.4–4.0 mIU/L, FT4 0.8–1.8 ng/dL (≈10–23 pmol/L), FT3 2.3–4.2 pg/mL (≈3.5–6.5 pmol/L). High TSH + low FT4 → overt primary hypothyroidism; high TSH + normal FT4 → subclinical hypothyroidism; low TSH + high FT4/FT3 → overt primary hyperthyroidism; low TSH + normal FT4 → subclinical hyperthyroidism, exogenous hormone, or resolving thyroiditis (check FT3 for T3 toxicosis); low/normal TSH + low FT4 → central hypothyroidism; high TSH + high FT4 → TSH-secreting adenoma or thyroid hormone resistance. Low FT3 with normal TSH/FT4 indicates low-T3 (non-thyroidal illness) syndrome. Units selectable for TSH, FT4, FT3. Derived from ATA 2014/2016, NACB 2002, Garber 2012, and MDCalc. Not medical advice.

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Key facts

Category
Health
Input types
select, number
Output type
json
Sample coverage
4
API ready
Yes

Overview

Thyroid Index interprets TSH with FT4 and optional FT3 to identify common thyroid-function patterns using adult reference ranges. Enter laboratory values, select the units, and receive a concise JSON result with the interpreted category, explanation, and reference range. This tool is for informational use and is not medical advice.

When to use

  • Review a TSH and FT4 combination for patterns consistent with hypo- or hyperthyroidism.
  • Add FT3 when TSH is low and FT4 is normal or when low-T3 syndrome or T3 toxicosis is a concern.
  • Compare results entered in supported units, including mIU/L or µIU/mL for TSH, ng/dL or pmol/L for FT4, and pg/mL or pmol/L for FT3.

How it works

  1. 1Select the reporting units for TSH, FT4, and FT3.
  2. 2Enter the required TSH and FT4 values, and optionally add FT3.
  3. 3The tool compares the values with adult reference ranges: TSH 0.4–4.0, FT4 0.8–1.8 ng/dL, and FT3 2.3–4.2 pg/mL.
  4. 4It returns a JSON interpretation containing the input values, category, explanation, and reference range.

Use cases

  • Clinicians and trainees can review common TSH, FT4, and FT3 patterns during endocrine education.
  • Patients can organize thyroid laboratory values before discussing them with a healthcare professional.
  • Researchers and health-content teams can generate consistent JSON interpretations from standardized thyroid test inputs.

Examples

1. Reviewing overt primary hypothyroidism

Patient preparing for a clinician visit

Background

A laboratory report shows a TSH of 12.0 mIU/L and FT4 of 0.5 ng/dL.

Problem

The patient wants a clear pattern-based summary of the elevated TSH and low FT4.

How to use

Enter TSH as 12, select mIU/L, enter FT4 as 0.5, select ng/dL, and leave FT3 blank.

TSH: 12 mIU/L; FT4: 0.5 ng/dL; FT3: not entered; decimal places: 2

Outcome

The JSON result identifies the pattern as overt primary hypothyroidism and explains that high TSH with low FT4 supports this interpretation.

2. Assessing low TSH with normal FT4

Clinician reviewing thyroid tests

Background

A patient has TSH of 0.1 mIU/L, FT4 of 1.2 ng/dL, and FT3 of 3.4 pg/mL.

Problem

The clinician needs a concise interpretation of suppressed TSH without elevated FT4 or FT3.

How to use

Enter the three laboratory values with TSH in mIU/L, FT4 in ng/dL, and FT3 in pg/mL.

FAQ

What values does the Thyroid Index use?

It uses TSH, FT4, and optional FT3 values with selectable units. The adult reference ranges are TSH 0.4–4.0 mIU/L, FT4 0.8–1.8 ng/dL, and FT3 2.3–4.2 pg/mL.

Is FT3 required?

No. TSH and FT4 are required; FT3 is optional and can help assess T3 toxicosis or low-T3 patterns.

What does high TSH with low FT4 indicate?

This pattern is interpreted as overt primary hypothyroidism based on the stated adult reference ranges.

What does low TSH with normal FT4 indicate?

This pattern is interpreted as subclinical hyperthyroidism, exogenous hormone exposure, or resolving thyroiditis. FT3 can help identify T3 toxicosis.

Can this tool diagnose a thyroid condition?

No. It provides pattern-based informational interpretation and does not replace clinical assessment, laboratory-specific ranges, or medical advice.

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TSH: 0.1 mIU/L; FT4: 1.2 ng/dL; FT3: 3.4 pg/mL; decimal places: 2

Outcome

The JSON result identifies subclinical hyperthyroidism and notes that exogenous hormone or resolving thyroiditis may also fit the pattern.