Thyroid
Thyroxine-bindend globuline
The protein that carries thyroid hormone in the blood; an abnormal value does not change the free, active hormone.
Everything about this testWhat it measures
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.
- 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
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.
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.
If your result is abnormal
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
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.
Limitations
- 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.
How common is this
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.
Who is this 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.
Downsides 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.
Sources
Always discuss an abnormal result with your GP. If a result is strongly abnormal, we try to reach you as soon as possible. Read your result yourself as well and contact your GP if in doubt. A test does not make a diagnosis. Read more about risks and results