Thyroxine-binding globulin blood test in Chiswick

What the test measures

Most thyroid hormone in the bloodstream is bound to carrier proteins, and thyroxine-binding globulin (TBG) carries the largest share. Only the small unbound, or free, portion is active. When TBG levels change, total T4 and total T3 change too, even though the thyroid itself may be working normally.

TBG is not a first-line thyroid test. It is mainly used to explain unusual total thyroid hormone results. Levels rise with oestrogen, for example in pregnancy or with the combined pill or HRT, and with some liver conditions. Levels can fall with androgens, steroids, significant illness, kidney conditions that cause protein loss, or inherited variations in TBG.

The price includes the sample taken at our Chiswick clinic, analysis at the Randox accredited laboratory, a practitioner review of your result, and a GP feedback report on request.

Who it is for

• Your total T4 or total T3 result does not match your TSH or free hormone results.
• You are pregnant, or take oestrogen-containing contraception or HRT, and your thyroid results need interpreting.
• You have tiredness, weight change or feel cold and are reviewing your thyroid with a practitioner.
• A relative has a known inherited TBG variation.
• Your doctor has asked for TBG as part of a detailed thyroid assessment.

Understanding your result

Your report shows your TBG level and the laboratory reference range. A raised TBG can make total thyroid hormones look high, and a low TBG can make them look low, while free T4 and TSH stay normal. This is why TBG is read together with a full thyroid profile.

A TBG result on its own does not show whether you have an overactive or underactive thyroid. A value outside the range is a prompt for discussion with your practitioner and GP rather than a diagnosis.

Preparing for your test

You can eat and drink as usual beforehand. Please tell us if you are pregnant, or take the contraceptive pill, HRT, testosterone, steroids or thyroid medicine. Stop biotin supplements for at least 48 hours beforehand, as biotin can interfere with thyroid-related immunoassays.

TBG is most useful alongside TSH and free thyroid hormones, which you can find on our thyroid blood tests page. TBG also features in our broader health checks, including The Complete Woman and The Complete Man. See every blood test and price in Chiswick.

Questions

How much is a thyroxine-binding globulin test in Chiswick?

The Thyroxine-Binding Globulin test costs £149. The price covers the blood sample at our Chiswick clinic, analysis at the Randox accredited laboratory, a practitioner review of your result, and a GP feedback report on request.

Why would my TBG be high?

Oestrogen from pregnancy, the contraceptive pill or HRT is the most common reason. Some liver conditions and inherited variations can also raise TBG.

Does an abnormal TBG mean I have a thyroid problem?

Not necessarily. Changes in TBG alter total hormone levels but often leave free T4 and TSH normal, which means the thyroid may be working well.

Do I need TBG if I have had a normal TSH?

Usually not. TBG is mostly helpful when total thyroid hormone results look unusual.