Thyroid:- Part 4 – Thyroxine Binding Prealbumin (TBPA), Transthyretin, Thyretin
Thyroxine Binding Prealbumin
Sample for Thyroxine Binding Prealbumin
- The serum of the patient is needed.
- Analyze the sample fresh or store it at 4 °C for <72 hours.
- At -20 °C, the serum is stable for 6 months.
- Avoid serum with lipemia or hemolysis.
Purpose of the test (Indications)
- Transthyretin is advised to differentiate hyper or hypothyroidism.
- Genetic variants Transthyretin are associated with decreased or increased T4 and T3.
- Prealbumin estimation Is advised to work up malnutrition.
- Prealbumin is a better indicator of liver function than albumin.
Definition of Thyroxine Binding Prealbumin (Transthyretin)
- Prealbumin is synthesized in the liver and is found in the blood and CSF.
- Prealbumin is the fastest-migrating protein in serum electrophoresis. But not seen on the electrophoresis, and there is a very faint band.
- Prealbumin binds the thyroxine, so-called thyroxine-binding prealbumin (TBPA).
- Thyroxine-binding prealbumin has a molecular weight of 50,000 D.
- Transthyretin is known as prealbumin binding the thyroid hormones.
- Transthyretin is prealbumin that carries the thyroid hormones (T4 and T3) in the blood and CSF.
- So it transports thyroxine and retinol.
Thyroxin binding prealbumin (Transthyretin) interpretations:
- T4 and T3 are poorly soluble in the plasma, so they are transported by thyroglobulin and 15% by albumin and prealbumin.
- 10% of thyroid hormones are transported by the prealbumin.
- Thyroxine is 99% bound to Thyroid binding globulin, albumin, and prealbumin.
- Transthyretin, known as prealbumin, has a half-life of 48 hours (1.9 days).
- This TBPA does not play a major role in thyroid hormone functions.
- Prealbumin is secondary to thyroxine-binding globulin (TBG) for transporting T3 and T4.
- Zinc is needed for the synthesis of prealbumin. In the case of a low Zinc level, prealbumin will also be lower in amount.
- Prealbumin also affects the transportation and metabolism of vit. A.
Role of the transthyretin (prealbumin) in thyroid diseases:
- Transthyretin binds the T4, so changes in the Transthyretin level can affect the amount of T4 in the body’s tissues.
- Low Transthyretin level can lead to an increase in the T4 level, and end result is hyperthyroidism.
- In case of an increased level of transthyretin leads to decreased T4 level (free T4), and end result is hypothyroidism.
- Previously albumin was the marker of malnutrition now replaced by prealbumin.
- The albumin half-life is 21 days, while the half-life of prealbumin is 2 days (1.9 days).
- Its short half-life is the more sensitive indicator of any change affecting protein synthesis.
- This is the marker in the case of parenteral nutrition.
- There will be a marked decrease in the prealbumin level in liver diseases.
- Prealbumin will be decreased in inflammation, malignancies, and protein-losing diseases of the kidneys and intestine, so this is a negative acute-phase protein reactant.
- Prealbumin will be raised in Hodgkin’s lymphoma and chronic kidney diseases.
- Prealbumin can cross the blood-brain barrier, so it is found in the CSF.
Normal Thyroxine Binding Prealbumin (Transthyretin)
Transthyretin (Thyroxine-binding prealbumin)
|Adult||10 to 40|
Normal Prealbumin level:
- Adult/elders = 15 to 36 mg/dL.
- Children <5 days = 6 to 21 mg/dL.
- 1 to 5 years = 14 to 30 mg/dL.
- 6 to 9 years = 15 to 33 mg/dL.
- 10 to 13 years = 22 to 36 mg/dL.
- 14 to 19 years = 22 to 45 mg/dL
- Urine = 0.017 to 0.047 mg/day
- CSF = roughly 2% of the total CSF protein.
Increased Thyroxine-binding prealbumin (Transthyretin) seen in:
- In the case of high doses of corticosteroids.
- Nonsteroidal anti-inflammatory drugs.
- Hodgkin’s disease.
- Adrenal hyperactivity.
Decreased Tryroxin-binding prealbumin (Transthyretin) seen in:
- It is a very sensitive negative acute-phase protein. It may drop to <20% of the median level.
- It falls during calorie-protein malnutrition.
- It is also seen in several forms of hereditary amyloidosis.
Increase the level of Prealbumin seen in:
- Hodgkin’s disease.
- Nephrotic syndrome (some of the cases).
- Anabolic steroids, androgens, estrogen, and prednisone may increase the value.
Decreased level of Prealbumin seen in:
- Liver diseases.
- Drugs may decrease the level of oral contraceptives and amiodarone.
- The critical value of prealbumin is <10.7 mg/dL indicating severe malnutrition.
Questions and answers:
Question 1: What is the site of prealbumin synthesis and its function?
Prealbumin is synthesized in the liver, and its role is to transport thyroid hormone in the blood.
Question 2: Can we see prealbumin on electrophoresis?
It gives a faint band on electrophoresis.