Thyroid:- Part 3 – Thyroxine Binding Globulin (TBG)
Thyroxine Binding Globulin (TBG)
Sample for Thyroxine Binding Globulin
- Venous blood is needed to prepare the serum.
Purpose of the test (Indications) for Thyroxine Binding Globulin
- Help to evaluate patients who had abnormal T4 and T3.
- Diagnose hyperthyroidism with raised T4.
- To diagnose hereditary deficiency or increased TBG.
- Also helpful for the diagnosis of hypothyroidism.
- Some times used to detect recurrent or metastatic differentiated thyroid carcinoma, especially follicular carcinoma with an increased level due to carcinoma.
- It is advised to distinguish increased or decreased total T3 or total T4 concentration due to changes in TBG.
Definition of Thyroxine Binding Globulin (TBG)
- Thyroxine-binding globulin is a protein synthesized in the liver that binds to the thyroid hormones.
- Thyroxine-binding globulin is the protein that binds with thyroxine, specifically to T4 and, to a lesser extent, T3.
- Thyroxine-binding globulin is a carrier of thyroid hormones and carries them to various parts of the body.
- In summary, Thyroxine-binding globulin protein is a carrier protein.
- While Thyroglobulin is the protein produced in the thyroid follicles and takes part in the synthesis of thyroxine.
Thyroxine-binding globulin (TBG) interpretations:
- Almost all thyroid hormones are protein-bound.
- Thyroid hormones are bound to:
- Prealbumin (Thyroid binding prealbumin).
- Thyroxine-binding globulin is the most important.
- T4-binding protein is alpha-1- globulin
- TBG effect free and bound T4 and T3.
- T4 + T3 assay = total T4 and T3 = These measures bound and free thyroid hormone.
- Most of these hormones are bound to TBG.
- Free T3/T4 are metabolically active hormones.
- When there is an increase in TBG, more T4/T3 is bound, and less active free T4/T3 will be available, leading to the Stimulation of TSH that will produce more T4/T3. But there is no hypothyroidism because this is compensation due for increasing TBG. This is seen in pregnant women and patients taking exogenous estrogen.
- In the case of raised T4, evaluate that:
- Is it due to hyperthyroidism?
- Or due to increased TBG.
- Estrogen increases the TBG in the serum, whereas androgens and glucocorticoids decrease TBG synthesis.
Normal Thyroxine binding globulin (TBG)
|Cord blood||3.6 to 9.6|
|4 to 12 months||3.1 to 5.6|
|1 to 5 years||2.9 to 5.4|
|5 to 10 years||2.5 to 5.0|
|10 to 15 years||2.1 to 4.6|
Male = 1.2 to 2.5
Female = 1.4 to 3.0
|Pregnancy 3rd trimester||5.3 ± 0.6|
- To convert into SI unit x 10 = mg/L
|Age||Male mg/dL||Female mg/dL|
|1 to 5 days||2.2 to 4.2||2.2 to 4.2|
|1 to 11 months||1.6 to 3.6||1.7 to 3.7|
|1 to 9 years||1.2 to 2.8||1.5 to 2.7|
|10 to 19 years||1.4 to 2.6||1.4 to 3.0|
|>20 years||1.7 to 3.6||1.7 to 3.6|
|Oral contraceptive||1.5 to 5.5|
|Pregnancy (rd trimester)||4.7 to 5.9|
Thyroxine-binding globulins :
- Infants = 3 to 6 mg/dL.
- Men = 1.2 to 2.5 mg/dL.
- Women = 1.4 to 3.0 mg/dL.
- Female on Oral contraceptives = 1.5 to 5.5 mg/dL.
- Female Pregnancy (3rd trimester) = 4.7 to 5.9 mg/dL.
- Thyroxine binding globulin for T 4 = 10 to 25 mg/dL
- Prealbumin for T 4 = 49 to 70 mg/dL.
- Albumin for T 4 = 12 to 34 mg/dL
Increased Thyroxine binding globulin (TBG) level is seen in the:
- In some cases of hypothyroidism.
- Genetic abnormality.
- Lymphocytic painless subacute thyroiditis.
- Hepatitis (Infectious type)
- Porphyria (acute intermittent).
- Estrogen-producing tumors.
- Late HIV infection.
- Certain drugs like estrogen, birth control tablets, clofibrate, heroin, perphenazine, and methadone.
Decreased Thyroxine binding globulin (TBG) level is seen in:
- Genetic deficiency.
- Nephrotic syndrome.
- Protein-losing enteropathy.
- Testosterone-producing tumors where testosterone decreases TBG.
- Ovarian failure.
- Stress illness and surgical stress.
- Chronic liver disease.
Questions and answers:
Question 1: What is the difference between thyroxine-binding globulin and thyroglobulin?
Thyroxine binding globulin is a carrier protein for T4 and T3. While thyroglobulin is important for the synthesis of T4 and T3.
Question 2: What is the effect of estrogen on thyroxine-binding globulin?
Estrogen increases the TBG.