Thyroid Function Assessment Guidelines

Diagnostic Criteria

• The American College of Medical Genetics emphasizes using assay-specific ranges to avoid misdiagnosis, as each laboratory must use the reference range established by their specific analyzer and reagent system, because FT4 reference ranges vary significantly between different immunoassay platforms and manufacturers 5, 6
• FT4 should always be interpreted in conjunction with TSH levels for comprehensive thyroid assessment, as in congenital hypothyroidism of thyroidal origin, TSH is elevated with low FT4, while central hypothyroidism shows low FT4 without clearly elevated TSH 5, 6

Clinical Considerations

• The American Association of Clinical Endocrinologists recommends using the laboratory's established reference range for FT4, typically without age adjustment in adults 6
• Amiodarone can alter thyroid hormone levels, and non-thyroidal illness can cause abnormal thyroid function tests without actual thyroid dysfunction, while iodine exposure from CT contrast can transiently impact thyroid function tests 6