Table 1.

Differential diagnosis of hyperthyroxinemia with normal or increased TSH.

Diagnosis	Total T4	Free T4	TSH	Comments
Resistance to thyroid hormone	↑	↑	Normal or ↑	Defect in THRB gene
TSH-secreting pituitary tumor	↑	↑	↑	α subunit of TSH (SU) ↑ ( α SU/TSH > 1)
Familial dysalbuminemic hyperthyroxinemia	↑	↑ or normal	Normal	Mutant albumin gene
Medications (amiodarone)	↑	↑	Normal	Inhibits T4 → T3
Deiodinase defect	↑	↑	Normal or ↑	SBP2 gene mutation
Gain of function mutation of transthyretin (TTR)	↑	↑ or normal	Normal	Normal serum TTR levels
TBG excess	↑	Normal	Normal	High TBG concentration
Anti-T4 antibody (assay interference)	↑ or ↓	Normal, ↑ or ↓	Normal	Falsely high or low total T4