Thyroxine, total (total T4)

What it measures

Total T4 measures both free and protein-bound thyroxine. It is used where free hormone assays are unavailable and is influenced by changes in binding proteins.

Specimen and preparation

• Specimen: Serum or lithium-heparin plasma
• Tube: Gold top

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
• Raised TBG (pregnancy, oestrogens, oral contraceptives)
• Familial dysalbuminaemic hyperthyroxinaemia

Causes of low values

• Hypothyroidism
• Low TBG (nephrotic syndrome, androgens, liver disease)
• Severe illness

Method

Chemiluminescent or enzyme immunoassay

Good to know

Binding-protein changes can mislead; free T4 is generally preferred.

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