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.

Pathophysiology of Thyroxine Binding Globulin (TBG)

1. Thyroglobulin is a non-iodine-containing glycoprotein found in the colloid of the thyroid follicle.
   1. Thyroglobulin is the protein produced in the thyroid follicles and takes part in thyroxine formation.
   2. Thyroxine-binding globulins are the protein that binds with thyroxine.

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

4. TBG effect free and bound T4 and T3.
   1. T4 + T3 assay = total T4 and T3 = These are a measure of 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 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.
8. In the case of raised T4, evaluate that:
   1. Is it due to hyperthyroidism or?
   2. Due to increased TBG.
9. Estrogen increases the TBG in the serum whereas androgens and glucocorticoids decrease the TBG synthesis.

Normal

Thyroxine-binding globulin

Source 1

Age	mg/dL
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
Adult	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

Source 2

Thyroxine-binding globulin

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

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 TBG level is seen in:

1. In some cases of hypothyroidism.
2. Genetic abnormality.
3. Hepatitis (Infectious type)
4. Porphyria (acute intermittent).
5. Estrogen producing tumors.
6. late HIV infection.
7. Pregnancy.

Decreased 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.

---