HES1 and cyclic hematopoiesis: The present study aimed investigate the etiology of the permanent CH diagnosed, with the purpose of identifying the cases due to TD, and evaluate the role of four different candidate genes that have been suggested as being involved in thyroid embryogenesis: (paired box gene 8 (PAX-8), thyroid stimulating hormone receptor (TSH-R), transcription factor related locus 5 (NKX2-5) and hairy/enhancer of split 1 (HES-1) and having an influence on these outcomes.