# Thyroid Index (TSH/FT4/FT3 Interpretation) 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. > Canonical page: https://elysiatools.com/en/tools/thyroid-index - **Category:** Health - **Keywords:** thyroid index, TSH, free T4, FT4, free T3, FT3, thyroid interpretation, hypothyroidism, hyperthyroidism, subclinical hypothyroidism, subclinical hyperthyroidism, Graves, Hashimoto, central hypothyroidism, T3 toxicosis, endocrinology ## 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. ## Inputs - **TSH Unit** (select) - **FT4 Unit** (select) - **FT3 Unit** (select) - **TSH** (number): Thyrotropin (TSH). Unit selected under 'TSH Unit' (mIU/L or µIU/mL; numerically equivalent). - **Free T4 (FT4)** (number): Free T4. Unit selected under 'FT4 Unit' (ng/dL or pmol/L). - **Free T3 (FT3, optional)** (number): Optional free T3. Unit selected under 'FT3 Unit' (pg/mL or pmol/L). Used to detect T3 toxicosis and low-T3 syndrome. - **Decimal Places** (number) ## 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 - Select the reporting units for TSH, FT4, and FT3. - Enter the required TSH and FT4 values, and optionally add FT3. - The 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. - It 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. ## Frequently asked questions ### 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? 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