Total T4 (total thyroxine)

Thyroid · TT4 · also printed as Total T4, TT4, Total thyroxine, Thyroxine (total), T4 (total)

Typical UK adult range66 to 181 nmol/L

The range is higher in pregnancy and in people taking oestrogen-containing contraception because the binding proteins rise, and it varies between laboratories.

What this test measures

Total T4 measures all the thyroxine in the blood, both the large fraction bound to carrier proteins and the small free fraction. Because it is affected by how much carrier protein a person has, most UK laboratories now report free T4 instead. Total T4 is still printed on some older panels and on many overseas reports.

Why doctors request it

It is rarely requested on its own in the UK today. It appears on some older or overseas thyroid panels and is occasionally used when free T4 assays are thought to be unreliable, for example in some pregnancies or with unusual binding proteins.

What commonly pushes it up

Pregnancy and oestrogen-containing contraception or HRT, which raise the carrier proteins
An overactive thyroid
Taking too much thyroid hormone replacement
Some inherited differences in carrier proteins that are harmless
Some medicines, including amiodarone

What commonly pushes it down

An underactive thyroid
Low carrier proteins, for example with severe illness, liver disease or kidney protein loss
Some medicines, including anabolic steroids, anti-epileptics and high-dose salicylates
Not enough thyroid hormone replacement being taken

Everyday reasons for a value outside the range

It rises on the combined contraceptive pill and in pregnancy without any thyroid change
Inherited differences in carrier proteins change it without affecting free hormone
Laboratories use different assays with different ranges

About the sample

No fasting needed. It is best read together with TSH and, where available, free T4.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.

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This is an educational summary of results you uploaded. It is not medical advice, a diagnosis or a treatment plan, and it does not replace the doctor who requested the test. If you are worried, or a result is marked outside range, discuss it with your doctor. Written and reviewed by a UK consultant physician. Reference ranges vary between laboratories.