Thyroid Panel (Free T3, Free T4, TSH)
The Thyroid Panel measures thyroid-stimulating hormone (TSH), free thyroxine (Free T4), and free triiodothyronine (Free T3) in serum to provide a comprehensive assessment of thyroid function. TSH and Free T4 localise dysfunction through their feedback relationship, while Free T3 grades hyperthyroidism and detects T3 toxicosis.
3
Biomarkers
Blood (serum)
Sample
1–3 business days
Turnaround
No — however, practitioners should document timing relative to any thyroid medication, as free hormones peak hours after dosing
Fasting
What is the Thyroid Panel test?
The Thyroid Panel combines TSH, Free T4, and Free T3 to give a comprehensive biochemical assessment of thyroid function in a single draw. It establishes the central feedback relationship between the pituitary and the thyroid (TSH and Free T4) while adding the active hormone fraction (Free T3) for grading and characterising hyperthyroid states. Within thyroid assessment, TSH alone is the recommended first-line screen, with Free T4 added to confirm and localise dysfunction; Free T3 is most valuable in hyperthyroidism. Ordering all three together is appropriate when a fuller characterisation is wanted upfront — for example, in suspected hyperthyroidism, in evaluating discordant or complex thyroid presentations, in central thyroid disorders where TSH is unreliable, or in functional and integrative assessment where conversion of T4 to T3 is of interest. The panel localises dysfunction by pattern: concordant abnormalities point to primary thyroid disease, while discordant TSH–Free T4 patterns suggest central disorders, non-thyroidal illness, or assay interference.
What does the Thyroid Panel test measure?
TSH is a glycoprotein hormone secreted by the anterior pituitary under hypothalamic TRH control, regulating thyroid hormone synthesis and release. Its secretion is governed by negative feedback from circulating free thyroid hormones, with a log-linear relationship that makes TSH exquisitely sensitive to small changes in Free T4. TSH exhibits diurnal variation, peaking overnight and reaching a nadir in the afternoon, and has a circulating half-life of roughly an hour.
TSH is the most sensitive single marker of thyroid function and the recommended first-line screen for thyroid dysfunction in patients with an intact hypothalamic-pituitary axis. Because of feedback amplification, TSH moves before free hormones leave their reference range, allowing detection of subclinical disease. Its reliability depends on a normal pituitary axis; in central thyroid disorders TSH alone is misleading and must be paired with Free T4.
Thyroxine is the principal hormone secreted by the thyroid under TSH stimulation. Free T4 is the unbound fraction — approximately 0.03% of total T4 — that is metabolically available and serves as the substrate for peripheral deiodination to the more active T3. The bound majority circulates attached to TBG, transthyretin, and albumin, and the free fraction is independent of binding-protein concentration. Free T4 has a circulating half-life of approximately seven days.
Free T4 quantifies the thyroid hormone supply available to tissues and, paired with TSH, localises thyroid dysfunction. A low Free T4 with elevated TSH confirms primary hypothyroidism; an elevated Free T4 with suppressed TSH confirms hyperthyroidism. Free T4 is essential in suspected central thyroid disease, where TSH is unreliable, and has replaced Total T4 in routine practice because it is unaffected by binding-protein changes.
Triiodothyronine is the metabolically active thyroid hormone. About 80% derives from peripheral deiodination of T4, with the remainder secreted directly by the thyroid. Free T3 is the unbound fraction — roughly 0.3% of total T3 — available to bind nuclear thyroid hormone receptors, with the bound majority carried by TBG, transthyretin, and albumin. Free T3 has a short circulating half-life of approximately one day.
Free T3 reflects the receptor-active thyroid hormone pool and is most informative in grading hyperthyroidism and detecting T3-predominant disease (T3 toxicosis), where Free T3 rises while Free T4 may remain within range. It also falls in non-thyroidal illness through reduced peripheral conversion. Free T3 adds little to the diagnosis of hypothyroidism, where TSH and Free T4 are the first-line markers.
Why is the Thyroid Panel test ordered?
- Comprehensive assessment of suspected thyroid dysfunction where a full TSH–Free T4–Free T3 picture is wanted in a single draw
- Investigation of suspected hyperthyroidism, where Free T3 grades hormone excess and identifies T3-predominant disease that TSH and Free T4 alone may miss
- Evaluation of discordant or complex thyroid presentations requiring all three markers to clarify the pattern
- Suspected central (secondary) thyroid disorders, where TSH is unreliable and Free T4 is essential for interpretation
- Characterisation of suspected impaired T4-to-T3 conversion in functional and integrative thyroid assessment
- Monitoring of thyroid hormone replacement or antithyroid therapy where free hormone levels guide titration alongside TSH
- Assessment of thyroid status in patients with non-specific systemic symptoms where a single comprehensive panel is preferred to sequential testing
Sample collection and turnaround
Sample type
Blood (serum)
Fasting required
No — however, practitioners should document timing relative to any thyroid medication, as free hormones peak hours after dosing
Collection method
Venipuncture at patient service centre
Turnaround
1–3 business days
Collection notes
Document thyroid medication and dose timing, pregnancy status, and any glucocorticoid therapy. Advise patients to withhold high-dose biotin supplementation for 48–72 hours prior to collection to avoid immunoassay interference. Consistent collection time aids comparison of serial TSH results.
Specimen requirements
Minimum 1.0 mL serum. Separate serum from cells within 2 hours of collection. Specimen stable refrigerated; follow Quest transport guidelines.
What can affect Thyroid Panel results?
Biotin interferes with streptavidin-biotin immunoassays, distorting TSH and the free hormones in opposite directions and producing patterns that can mimic thyroid disease. Patients should withhold biotin for 48–72 hours before collection.
Systemic illness lowers Free T3 through reduced peripheral conversion and can variably alter TSH and Free T4, producing the low-T3 (euthyroid sick) pattern that does not reflect intrinsic thyroid disease.
Free T4 peaks 2–4 hours after a levothyroxine dose and Free T3 after liothyronine, so specimens drawn soon after dosing overestimate steady-state levels. Timing relative to the last dose should be documented.
TSH peaks overnight and falls to an afternoon nadir, so collection time can shift borderline results. Consistent timing aids comparison of serial measurements.
Raised TBG from oestrogen or pregnancy has little effect on free hormone assays but alters total hormone measurements; trimester-specific reference ranges are needed in pregnancy for reliable interpretation.
High-dose glucocorticoids and dopamine suppress TSH secretion, and several drugs alter thyroid hormone binding or conversion, complicating interpretation of the panel.
What do Thyroid Panel results mean?
Elevated
Elevated TSH with low Free T4 confirms primary hypothyroidism; elevated TSH with normal free hormones indicates subclinical hypothyroidism. Elevated Free T4 and/or Free T3 with suppressed TSH confirms hyperthyroidism, with isolated Free T3 elevation defining T3 toxicosis. Elevated free hormones with non-suppressed TSH are unusual and raise suspicion of a TSH-secreting adenoma, hormone resistance, or assay interference.
Normal
Normal TSH, Free T4, and Free T3 confirm euthyroid status in a patient with an intact pituitary axis. A normal TSH with abnormal free hormones, or vice versa, should prompt consideration of central disorders, assay interference, recent dose timing, or non-thyroidal illness.
Reduced
Low TSH with elevated free hormones indicates hyperthyroidism; low TSH with normal free hormones suggests subclinical hyperthyroidism. Low Free T4 with low or inappropriately normal TSH points to central hypothyroidism. A low Free T3 in isolation, with normal TSH and Free T4, most often reflects non-thyroidal illness.
Note: The panel is interpreted as a pattern, not as isolated values, with TSH and Free T4 anchoring the assessment and Free T3 adding information chiefly in hyperthyroidism. TSH is unreliable in central disorders and is suppressed by some drugs; biotin, dose timing, and systemic illness can all distort the picture. Free hormone assays are method-dependent at the extremes.
Frequently asked questions
It measures TSH, Free T4, and Free T3 together. TSH and Free T4 establish the pituitary-thyroid feedback relationship and localise dysfunction, while Free T3 adds the active hormone fraction for grading hyperthyroidism.
How to access the Thyroid Panel test
The Thyroid Panel reference page is free and open to all practitioners. Create a free practitioner account to see pricing, save tests to your list, and generate requisition forms. Verification takes 1–2 business days.
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Published: June 2, 2026
Last reviewed: June 2, 2026
- Jonklaas J, et al. Guidelines for the Treatment of Hypothyroidism. Thyroid. (2014)
- Ross DS, et al. 2016 American Thyroid Association Guidelines for Diagnosis and Management of Hyperthyroidism and Other Causes of Thyrotoxicosis. Thyroid. (2016)
- Garber JR, et al. Clinical practice guidelines for hypothyroidism in adults. Thyroid. (2012)
- Sheehan MT. Biochemical Testing of the Thyroid: TSH is the Best and, Oftentimes, Only Test Needed. Clin Med Res. (2016)
This content is for licensed healthcare practitioners only and does not constitute medical advice. Clinical decisions should be based on the full clinical picture and not on any single test result.
Related tests
thyroid endocrine
TSH
The TSH test measures Thyroid Stimulating Hormone produced by the anterior pituitary gland. It is the recommended first-line screening test for thyroid dysfunction, with elevated TSH indicating primary hypothyroidism and suppressed TSH indicating hyperthyroidism. TSH alone does not assess T4-to-T3 conversion or exclude central hypothyroidism.
thyroid endocrine
Free T4
The Free T4 test measures free thyroxine, the unbound and metabolically available fraction of T4, in serum. Interpreted alongside TSH, it confirms and localises thyroid dysfunction — diagnosing primary and central hypothyroidism, grading hyperthyroidism, and monitoring levothyroxine therapy. Free T4 has replaced Total T4 in routine practice because it is unaffected by binding-protein changes.
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Free T3
The Free T3 test measures free triiodothyronine, the unbound and biologically active fraction of T3, in serum. It is primarily indicated for assessing the severity of hyperthyroidism, detecting T3 toxicosis, and characterising non-thyroidal illness syndrome. TSH and Free T4 remain the first-line markers for thyroid screening and hypothyroidism.