High serum thyroxine-binding globulin--an important cause of hyperthyroxinaemia.

Area of Science:

• Endocrinology
• Clinical Chemistry

Background:

• Thyrotoxicosis diagnosis relies on thyroid hormone levels, but elevated thyroxine-binding globulin (TBG) can complicate interpretation.
• Significant TBG elevation is an uncommon but notable finding in hospital populations.

Observation:

• Four patients presented with significantly elevated serum TBG levels.
• Three of these patients received inappropriate therapy for thyrotoxicosis due to misinterpretation of thyroid function tests.
• Elevated TBG is present in approximately 0.45% of the hospital population.

Findings:

• Patients with elevated TBG exhibit serum thyroxine levels comparable to those with clinical thyrotoxicosis.
• Tri-iodothyronine levels are also frequently elevated, though typically less so than in overt thyrotoxicosis.
• The thyrotropin-releasing hormone (TRH) test proved valuable in assessing thyroid status in these patients.

Implications:

• Measurement of TBG is indicated in all cases of hyperthyroxinemia with doubtful clinical features of thyrotoxicosis.
• Accurate TBG measurement can prevent misdiagnosis and inappropriate treatment for thyroid disorders.
• Utilizing the TRH test aids in differentiating true hyperthyroidism from TBG-induced elevations.