Thyroid – Part 4 – Thyroxine Binding Prealbumin (TBPA), Transthyretin, Thyretin

Sample

1. The serum of the patient is needed.
2. Analyze the sample fresh or store at 4 °C for <72 hours.
3. At -20 °C serum is stable for 6 months.
4. Avoid serum with lipemia or hemolysis.

Purpose of the test (Indications)

1. This test is advised to work up malnutrition.
2. This is a better indicator of liver function than the albumin.
3. Genetic variants Transthyretin associated with decreased or increased T4 and T3.

Pathophysiology

1. This is the fastest migrating protein in the serum electrophoresis. But not seen on the electrophoresis and there is a very faint band.
2. Prealbumin binds the thyroxine so-called as thyroxine-binding prealbumin (TBPA).
   1. Transthyretin is known as prealbumin binding the thyroid hormones.
      1. Transthyretin is prealbumin which carries the thyroid hormones  (T4 and T3) in the blood and CSF.
      2. So it transports the thyroxine and retinol.
   2. T4 and T3 poorly soluble in the plasma so transported by thyroglobulin and 15% by albumin and prealbumin.
   3. 10% of thyroid hormones are transported by the prealbumin.

1. 4. Thyroxine 99% bound to Thyroid binding globulin, albumin, and prealbumin.
      1. Transthyretin knows as prealbumin has a half-life of 48 hours (1.9 days).
      2. This TBPA does not play a major role in thyroid hormones functions.
   5. Prealbumin is secondary to thyroxine-binding globulin (TBG) for the transportation of T3 and T4.

3. Zinc is needed for the synthesis of prealbumin. In the case of a low Zinc level, prealbumin will also be lower in amount.
   1. Prealbumin also role in the transportation and metabolism of vit.A.
4. Previously albumin was the marker of malnutrition now replaced by prealbumin.
5. Albumin half-life is 21 days, while the half-life of prealbumin is 2 days (1.9 days).
   1. Because of its short half-life is the more sensitive indicator of any change affecting the protein synthesis.
   2. This is the marker in the case of parenteral nutrition.
   3. There will be a marked decrease in the prealbumin level in liver diseases.
   4. 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.
6. Prealbumin will be raised in Hodgkin’s lymphoma and chronic kidney diseases.
7. Prealbumin can cross the blood-brain barrier, so it is found in the CSF.

Normal

Source 1

Transthyretin (Thyroxine-binding prealbumin)

Age	mg/dL
Cord blood	13
One year	10
Maternal	23
Adult	10 to 40
Male	21.5
Female	18.2

Source 2
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:

1. In case of high doses of corticosteroids.
2. Nonsteroidal anti-inflammatory drugs.
3. Hodgkin’s disease.
4. Adrenal hyperactivity.

Decreased Tryroxin-binding prealbumin (Transthyretin) seen in:

1. It is a very sensitive negative acute-phase protein. It may drop to <20% of the median level.
2. It falls during calorie-protein malnutrition.
3. It is also seen in several forms of hereditary amyloidosis.

Increase level of Prealbumin seen in:

1. Hodgkin’s disease.
2. Nephrotic syndrome (some of the cases).
3. Pregnancy.
4. Anabolic steroids, androgens, estrogen, and prednisone may increase the value.

Decreased level of Prealbumin seen in:

1. Malnutrition.
2. Liver diseases.
3. Inflammation.
4. Burns.
5. Drugs may decrease the level of oral contraceptives and amiodarone.

• The critical value of prealbumin is <10.7 mg/dL indicate severe malnutrition.

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