Thyroid dyshormonogenesis usually is caused by single gene mutations, encoding globulins involved in TH synthesis (thyroid peroxidase - TPO, thyroglobulin - TG, Na+/I- symporter – SCL5A5/NIS, pendrine – SCL26A4/PDS), or directly influencing TH synthesis and metabolism (DUOX2, DUOX2A, IYD/DEHAL1) (6). This evidence concerns the gene SLC5A5 and familial thyroid dyshormonogenesis.