[Determining serum TBG level using the charcoal-cellulose I-125 thyroxine uptake test]

Area of Science:

• Biochemistry
• Endocrinology
• Radiochemistry

Context:

• Thyroxine-binding globulin (TBG) is a key protein in thyroid hormone transport.
• Accurate measurement of TBG is essential for evaluating thyroid status, particularly in cases of euthyroidism with abnormal TBG levels.
• Existing methods for TBG determination can be complex or inaccessible.

Purpose:

• To present a novel, accessible radiocompetition method for quantifying blood serum TBG levels.
• To validate the utility of the determined TBG values in calculating the free thyroxine index (FTI).
• To demonstrate the method's effectiveness in differentiating various thyroid functional states.

Summary:

• A radiocompetition assay utilizing 125I-thyroxine, cellulose, and activated charcoal has been developed for TBG determination.
• The method involves saturating TBG with L-thyroxine at a specific concentration (2.9 nmol/l) before radioligand binding.
• This technique is comparable to existing labeled thyroxine uptake methods in terms of reagents and equipment but offers simplicity and accessibility.

Impact:

• The calculated free thyroxine index (TT4/TBG) effectively distinguishes thyroid functional states.
• This method provides a reliable measure of free thyroid hormones, even with variable TBG concentrations in euthyroid individuals.
• The accessibility of the method makes it suitable for routine use in radiochemical laboratories for improved thyroid diagnostics.