Plasma free thyroxine concentrations in patients receiving levothyroxine for thyroid suppression

Area of Science:

• Endocrinology
• Thyroidology
• Clinical Chemistry

Background:

• Levothyroxine (L-thyroxine) is a common treatment for thyroid disorders.
• Current recommendations often focus on monitoring total triiodothyronine (T3) during L-thyroxine therapy.
• Elevated total thyroxine (T4) with normal T3 is observed in many euthyroid patients on L-thyroxine.

Purpose of the Study:

• To investigate whether free thyroxine (FT4), the active form of T4, is elevated in clinically euthyroid patients receiving L-thyroxine.
• To compare FT4 levels measured by analog radioimmunoassay methods in patients on L-thyroxine versus controls.

Main Methods:

• Prospective study of 27 clinically euthyroid patients on L-thyroxine and 21 euthyroid controls.
• Analysis of serum samples for T4, T3, T3 (RU), TSH, and FT4 using three different analog methods.
• Comparison of FT4 levels and the percentage of patients with FT4 in the hyperthyroid range between groups.

Main Results:

• Mean FT4 levels were significantly higher in the L-thyroxine group compared to controls across all three assays.
• A substantial percentage of patients on L-thyroxine had FT4 levels in the hyperthyroid range (41-63%).
• T3 concentrations did not differ significantly between groups, while thyrotropin levels were lower in the L-thyroxine group.

Conclusions:

• Free thyroxine (FT4) levels, when measured by analog methods, are frequently in the hyperthyroid range in clinically euthyroid patients on L-thyroxine.
• Relying on FT4 for thyroid status assessment may lead to inappropriate L-thyroxine dose adjustments.
• Total triiodothyronine (T3) levels remain a more reliable indicator of peripheral metabolic status and clinical impression.