Free thyroxine (free T4)

What it measures

Free T4 is the unbound, active fraction of the main thyroid hormone. It is measured with TSH to confirm and grade thyroid dysfunction and to monitor treatment, especially in pituitary disease.

Specimen and preparation

• Specimen: Serum or lithium-heparin plasma
• Tube: Gold top
• Preparation: Take the sample before the daily levothyroxine dose; avoid biotin supplements.

Reference range

Reference ranges differ between laboratories, methods and populations. Always use the range printed on your own lab report.

Causes of high values

• Hyperthyroidism
• Levothyroxine taken shortly before sampling
• Amiodarone
• Heparin (in vitro artefact)

Causes of low values

• Primary hypothyroidism
• Central (pituitary) hypothyroidism
• Severe non-thyroidal illness
• Phenytoin and carbamazepine

Method

Chemiluminescent immunoassay (analogue or two-step)

Good to know

Use trimester-specific or method-specific ranges in pregnancy.

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