Thyroid – Part 3 – Thyroxine Binding Globulin (TBG), Thyroid Binding Globulin
- Venous blood is needed to prepare the serum.
Purpose of the test (Indications)
- 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.
- Thyroglobulin is a non-iodine containing glycoprotein found in the colloid of the thyroid follicle.
- Thyroglobulin is the protein produced in the thyroid follicles and takes part in the thyroxine formation.
- Thyroxine binding globulins are the protein that binds with thyroxine.
- Almost all the thyroid hormones are protein-bound.
- Thyroid hormones are bound to:
- Prealbumin (Thyroid binding prealbumin).
- Thyroid binding globulin is most important.
- TBG effect free and bound T4 and T3.
- T4 + T3 assay = total T4 and T3 = These are a measure of 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 TBG then more T4/T3 bound to less active T4/T3 will be available which will lead to the Stimulation of TSH produce more T4/T3. But there is no hypothyroidism because this is compensation due to 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 the TBG synthesis.
|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 TBG level is seen in:
- In some cases of hypothyroidism.
- Genetic abnormality.
- Hepatitis (Infectious type)
- Porphyria (acute intermittent).
- Estrogen producing tumors.
- late HIV infection.
Decreased 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.