Thyroid and Endocrine Test Pitfalls

Expansion

Timing

• Cortisol has a diurnal rhythm; a 9 am sample is required
• Testosterone should be measured in the morning
• Prolactin rises with stress and venepuncture, so a cannulated resting sample may be needed
• Female hormones must be interpreted against the cycle day

Binding proteins: pregnancy and oestrogens raise thyroxine-binding globulin and cortisol binding globulin, raising total but not free hormone. Measure free hormone.

Assay interference

• Biotin supplements interfere with many immunoassays, causing falsely low TSH and falsely high free T4, mimicking thyrotoxicosis
• Macroprolactin gives a falsely raised prolactin
• The hook effect gives a falsely low result at extremely high concentrations

Acute illness: the sick euthyroid pattern makes thyroid tests uninterpretable, and cortisol is raised as part of the stress response, so a normal value in shock may still represent relative insufficiency.

Drugs: steroids suppress the adrenal axis; amiodarone, lithium and immune checkpoint inhibitors disturb thyroid function; opioids suppress the gonadal axis.