A solid-phase radioimmunoassay for free thyroxine in serum compared with equilibrium dialysis method

Area of Science:

• Endocrinology
• Clinical Chemistry

Background:

• Accurate measurement of serum free thyroxine (T4) is crucial for diagnosing thyroid disorders and understanding conditions affecting thyroxine-binding proteins.
• Existing methods for free T4 determination can be complex or less sensitive.

Purpose of the Study:

• To evaluate a novel solid-phase radioimmunoassay for serum free thyroxine (T4) measurement.
• To compare the assay's performance against established methods, particularly equilibrium dialysis.
• To assess the assay's utility across various thyroid states and in conditions with altered T4-binding proteins.

Main Methods:

• Serum free T4 was measured using a solid-phase radioimmunoassay.
• Results were compared with values derived from the product of total serum T4 and percent free T4 obtained by equilibrium dialysis.
• Assay performance was evaluated in normal subjects, hyperthyroid patients, hypothyroid patients, pregnant women, and patients with decreased thyroxine-binding globulin.

Main Results:

• The radioimmunoassay demonstrated high accuracy and sensitivity in measuring serum free T4.
• Mean serum free T4 levels were significantly different in hyperthyroid (5.02 +/- 1.41 ng/100 ml) and hypothyroid (0.16 +/- 0.15 ng/100 ml) patients compared to normal subjects (1.34 +/- 0.33 ng/100 ml).
• Free T4 levels were within the normal range in pregnant women and in most patients with decreased thyroxine-binding globulin.
• An excellent correlation (r = 0.93) was found between the radioimmunoassay and the equilibrium dialysis method.

Conclusions:

• The developed solid-phase radioimmunoassay is a simple, accurate, sensitive, and specific method for determining serum free T4.
• This assay is highly suitable for routine clinical determination of free thyroxine levels.
• The method effectively distinguishes various thyroid states and accounts for changes in T4-binding proteins.