Blood draw
TBG (Thyroxine-Binding Globulin)
TBG; thyroid hormone transport.
€ 41,00
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Thyroid · Blood
Thyroxine-binding globulin
The protein that carries thyroid hormone in the blood; an abnormal value does not change the free, active hormone.
At a glance
- Measured in blood
- In 1 test panel
- No special preparation
Explanation
Thyroxine-binding globulin: what is measured?
TBG (thyroxine-binding globulin) is made in the liver and binds most of the thyroid hormones T4 and T3 in the blood. Bound hormone is not active; only the small free part works. If the amount of TBG changes, total T4 changes with it, but free T4 stays the same and the thyroid works normally. That is why free T4 is measured when testing thyroid function.
What can it detect?
- An inborn shortage or excess of TBG, as an explanation for an abnormal total T4 with normal thyroid function
- The effect of pregnancy, the pill or estrogen use on total thyroid hormone
- In newborns: an explanation for a low total T4 in the heel prick screening
Result
What does the result mean?
Thyroxine-binding globulin too high
A raised TBG usually fits pregnancy or the use of the pill or other estrogens; tamoxifen, some other medicines and inflammation of the liver also raise the value. Sometimes a raised TBG is inborn. Total T4 is then higher, but free T4 and TSH stay normal and there are no symptoms.
Thyroxine-binding globulin too low
An inborn partial or complete shortage of TBG occurs and has no consequences for health; it is a natural variation. A low value also occurs with use of male hormones or anabolic agents and corticosteroids, with severe illness, malnutrition, liver disease and protein loss through the kidneys or bowel. Total T4 is then lower, but free T4 and TSH stay normal.
After an abnormal result
The GP assesses TSH and free T4. If these are normal, an abnormal TBG has no consequences and further tests or treatment are not needed. With an abnormal TSH or free T4, the usual tests for a thyroid disorder follow.
Reliability
How reliable is the test?
TBG says nothing about how well the thyroid works; according to the Dutch GP guideline (NHG) on thyroid disorders, that is determined with TSH and free T4. According to the Dutch Society for Clinical Chemistry (NVKC), the result means the most in newborns with a low total T4 in the heel prick screening. In adults, an abnormal TBG at most explains why a total T4 is abnormal while free T4 and TSH are normal. An abnormal TBG has no clinical meaning on its own.
What can affect the result?
- Pregnancy, the pill and other estrogens raise the value; male hormones, anabolic agents and corticosteroids lower it.
- Severe illness, malnutrition, liver disease and protein loss temporarily change the value.
- Reference values depend on age and sex and differ between laboratories.
Who it is for
Who is the test useful for?
Useful for
The result means the most as an explanation for an unexpectedly abnormal total T4 with a normal TSH and free T4, and in newborns with a low total T4 in the heel prick. With symptoms that fit a thyroid disorder, TSH and free T4 say more. In people without symptoms, a stand-alone value says little.
How common is it?
According to the Dutch Society for Clinical Chemistry (NVKC), a partial shortage of TBG occurs in about 1 in 200 people and a complete shortage in about 1 in 2500; neither causes symptoms. Thyroid disorders themselves are more common: an underactive thyroid in about 3 to 4 in 100 adults.
NVKC (allesovertesten.nl: TBG) and NIVEL primary care registrations
Drawbacks and risks
- An abnormal TBG, or a total T4 that is abnormal because of it, can wrongly be read as a thyroid problem, while the free, active hormone is normal.
Get tested
Thyroxine-binding globulin: test panels
Thyroxine-binding globulin is part of these test panels. You order online and have your blood drawn at a collection point or collect the sample at home.
Evidence
Sources
This explanation is based on Dutch guidelines and independent information. It is general information, not a diagnosis. Discuss an abnormal result with your GP.
More about risks and resultsMore explanations
Other tests: Thyroid
- Anti-Tg
- Anti-TPO
- Free T3
- Free T4
- Reverse T3
- T3 (total)
- Thyroglobulin
- Total T4
- TSH
- TSH receptor antibodies (TRAb)