What this test measures
This compact panel measures the two markers most international guidelines recommend for routine thyroid assessment:
• TSH (3rd Generation) — the pituitary signal that drives thyroid output. The single most sensitive marker of primary thyroid dysfunction. • Free T4 (FT4) — the biologically active thyroxine, providing the thyroid "output" reading without interference from carrier proteins.
T3 is omitted because, in most clinical situations (especially primary hypothyroidism), T3 adds little to a TSH + FT4 result — the body preserves T3 at the expense of T4 until late in the disease.
Why it matters
The American Thyroid Association, the Endocrine Society and Indian Thyroid Society all support TSH-first screening for routine thyroid assessment. Adding FT4 is helpful when the TSH is at the extremes, in pregnancy, with central hypothyroidism, in patients on levothyroxine and to confirm overt hypo- or hyperthyroidism.
This panel is the cost-effective default for routine outpatient thyroid screening, post-treatment monitoring, and antenatal thyroid checks. T3 / FT3 can be added as a separate test when hyperthyroidism is specifically suspected.
How to prepare
No fasting required. Morning sample preferred. If on levothyroxine, take it after the blood draw. Stop biotin for 48–72 hours. Continue other medications.
Markers & reference ranges
Reference ranges below are typical adult values. Your lab's reported range may differ slightly based on the assay platform and patient demographics — always read your report against the range printed on it.
| Marker | Normal range | If low | If high |
|---|---|---|---|
| Free Thyroxine (FT4) (ng/dL)[1] | 0.8 – 1.8 ng/dL (10 – 23 pmol/L) | Low FT4 with raised TSH = primary hypothyroidism. Low FT4 with low or inappropriately normal TSH = central hypothyroidism — needs pituitary workup. | Raised FT4 with suppressed TSH = primary hyperthyroidism. Pregnancy, oestrogen and biotin interfere with interpretation. |
| TSH (3rd Generation) (mIU/L)[1] | 0.4 – 4.0 (general adult); pregnancy-specific 1st trimester | Suppressed TSH = hyperthyroidism (overt or subclinical), over-replacement on levothyroxine, or biotin interference. Confirm with FT4. | Raised TSH = primary hypothyroidism. Mild rises (4–10 mIU/L) are subclinical and re-checked in 6–8 weeks. |
TSH + FT4 — pattern recognition
| TSH | FT4 | Likely picture |
|---|---|---|
| Raised | Low | Overt primary hypothyroidism — start levothyroxine |
| Raised | Normal | Subclinical hypothyroidism — re-check 6–8 weeks |
| Suppressed | Raised | Overt primary hyperthyroidism — refer |
| Suppressed | Normal | Subclinical hyperthyroidism or T3 toxicosis — add FT3 |
| Low / normal | Low | Central hypothyroidism — pituitary workup |
| Normal | Normal | Euthyroid |
Frequently asked questions
Why is T3 not included?
In most clinical situations, T3 adds little to the diagnosis — the body preserves T3 at the expense of T4 until late in hypothyroidism, and isolated T3 rise (T3 toxicosis) is uncommon. T3 / FT3 can be added if hyperthyroidism is specifically suspected.
Is this enough for a routine thyroid check?
Yes — TSH + FT4 is the panel most international guidelines (ATA, Endocrine Society, Indian Thyroid Society) endorse for routine screening, monitoring of replacement therapy, and pregnancy assessment.
Why use the "3rd generation" TSH?
Third-generation TSH assays measure down to 0.01 mIU/L, which is essential for detecting fully suppressed TSH (hyperthyroidism or over-replacement). Older assays could not distinguish "low" from "very low".
Do I need to fast?
No. Fasting is not required. A morning sample is preferred for consistency.
Will biotin affect the result?
Yes — high-dose biotin in hair-skin-nail supplements interferes with both TSH and FT4 immunoassays, typically giving falsely low TSH and falsely raised FT4. Stop 48–72 hours before.
My TSH is borderline — should I be worried?
A mildly raised TSH (4–10 mIU/L) with normal FT4 is "subclinical hypothyroidism". It is usually re-checked in 6–8 weeks. Most cases do not need treatment unless you are pregnant, symptomatic, or anti-TPO positive.
When would my doctor add antibodies (anti-TPO)?
If your TSH is raised, you are planning pregnancy, you have unexplained subfertility, or there is a family history of thyroid disease.
Related Hormones / Endocrine tests
Tests commonly ordered alongside Thyroid Panel 4, Thyroscreen FT4 And TSH-3Rd Gen, or that help interpret an unexpected result.
Sources & references
- American Thyroid Association — Thyroid Function Tests · accessed 2026-05-30T00:00:00.000Z
- Endocrine Society Clinical Practice Guidelines · accessed 2026-05-30T00:00:00.000Z
- NIH MedlinePlus — Thyroid Tests · accessed 2026-05-30T00:00:00.000Z
- Indian Thyroid Society — Consensus · accessed 2026-05-30T00:00:00.000Z
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