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

1. Help to evaluate patients who had abnormal T4 and T3.
2. Diagnose hyperthyroidism with raised T4.
3. To diagnose hereditary deficiency or increased TBG.
4. Also helpful for the diagnosis of hypothyroidism.
5. Some times used to detect recurrent or metastatic differentiated thyroid carcinoma, especially follicular carcinoma with an increased level due to carcinoma.
6. 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)

1. Thyroxine-binding globulin is a protein synthesized in the liver that binds to the thyroid hormones.
2. Thyroxine-binding globulin is the protein that binds with thyroxine, specifically to T4 and, to a lesser extent, T3.
3. Thyroxine-binding globulin is a carrier of thyroid hormones and carries them to various parts of the body.
4. In summary, Thyroxine-binding globulin protein is a carrier protein.
   1. While Thyroglobulin is the protein produced in the thyroid follicles and takes part in the synthesis of thyroxine.

Thyroxine-binding globulin (TBG) interpretations:

1. Almost all thyroid hormones are protein-bound.
2. Thyroid hormones are bound to:
   1. Albumin.
   2. Prealbumin (Thyroid binding prealbumin).
   3. Thyroxine-binding globulin is the most important.
3. T4-binding protein is alpha-1- globulin

4. TBG effect free and bound T4 and T3.
   1. T4 + T3 assay = total T4 and T3 = These measures bound and free thyroid hormone.
5. Most of these hormones are bound to TBG.
6. Free T3/T4 are metabolically active hormones.
7. 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.
8. In the case of raised T4, evaluate that:
   1. Is it due to hyperthyroidism?
   2. Or due to increased TBG.
9. Estrogen increases the TBG in the serum, whereas androgens and glucocorticoids decrease TBG synthesis.

Normal Thyroxine binding globulin (TBG)

Source 1

• To convert into SI unit x 10 = mg/L

Source 2

Thyroxine-binding globulin

Source 4

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:

1. In some cases of hypothyroidism.
2. Genetic abnormality.
   1. Neonates.
   2. Inherited.
   3. Idiopathic
3. Lymphocytic painless subacute thyroiditis.
4. Hepatitis (Infectious type)
5. Porphyria (acute intermittent).
6. Estrogen-producing tumors.
7. Late HIV infection.
8. Pregnancy.
9. Certain drugs like estrogen, birth control tablets, clofibrate, heroin, perphenazine, and methadone.

Decreased Thyroxine binding globulin (TBG) level is seen in:

1. Genetic deficiency.
2. Nephrotic syndrome.
3. Protein-losing enteropathy.
4. Malnutrition.
5. Testosterone-producing tumors where testosterone decreases TBG.
6. Ovarian failure.
7. Stress illness and surgical stress.
8. Chronic liver disease.
9. Acromegaly.

Questions and answers: