Interference in thyroid-stimulating hormone determination.

Area of Science:

• Clinical Chemistry
• Endocrinology
• Immunology

Background:

• Thyroid-stimulating hormone (TSH) is crucial for diagnosing thyroid disorders.
• Immunochemical TSH tests are generally reliable but susceptible to interference from various substances.
• Accurate TSH measurement is vital for appropriate patient management and treatment.

Observation:

• A 79-year-old man with hypothyroidism presented with pneumonia and an unexpectedly high TSH level (>100 mIU/L).
• Clinical evaluation and free thyroxine (FT4) levels did not indicate hyperthyroidism.
• Initial attempts to correct for heterophile antibodies did not normalize the TSH result.

Findings:

• The elevated TSH normalized when monoclonal protein bands, identified via immunofixation, disappeared from the patient's serum.
• Protein electrophoresis revealed abnormal IgG/j and IgM/k monoclonal bands.
• TSH levels measured by a different immunoassay method were normal, suggesting assay-specific interference.

Implications:

• Transient paraproteins may interfere with TSH immunoassays, causing falsely elevated results.
• This interference can mimic or mask true thyroid dysfunction, complicating diagnosis.
• Clinicians should consider paraproteinemia in cases of unexplained TSH abnormalities, especially when discordant with clinical findings.