Free T4

Free thyroxine — the small fraction of circulating T4 not bound to thyroid-binding proteins, freely available for tissue uptake and conversion to active T3. Used alongside TSH to confirm and characterise thyroid dysfunction.

Free T4 reflects the thyroid's hormone output directly, independent of binding-protein variation (which can confound total T4). Combined with TSH, it distinguishes primary thyroid disease (typical pattern: high TSH + low fT4 = overt hypothyroidism) from central (pituitary) causes.

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• Randox Health

  UK · EU · INTL

  Clinic-based premium panels — wider biomarker breadth than home-test brands.

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• Synlab

  DE · EU · INTL

  Europe-wide medical lab network — referrals via partner GPs and direct-to-consumer programmes where offered.

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• Cerascreen

  DE · EU

  Established German home-test catalogue — ISO-certified labs, German-language reports.

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• Medichecks

  UK

  UKAS-accredited home blood-test panels with GP-equivalent biomarker coverage.

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Reference ranges are highly assay-dependent — always interpret against the lab's reported range, not absolute numbers from another lab.

Severe non-thyroidal illness ('sick euthyroid syndrome') can lower fT4 without true thyroid disease.

• Whether your TSH and fT4 together suggest primary thyroid disease vs other patterns.
• Whether thyroid antibody testing would be useful for prognosis.

1. [1]Garber et al., AACE/ATA — Clinical practice guidelines for hypothyroidism(2014)