SLC16A2 and hyperthyroxinemia: We outline genetic or acquired conditions that are associated with genuine hyperthyroxinemia (eg, genetic or functional deficiency of deiodinase enzymes), hypertriiodothyroninemia (dyshormonogenesis, resistance to thyroid hormone α, monocarboxylate transporter 8 [MCT8] deficiency) or both raised FT4 and FT3 (Resistance to Thyroid Hormone β [RTHβ], TSH-secreting pituitary tumor).