Secondary CH may be: 1) hypothalamic (also called tertiary CH) – TRH gene mutations; 2) pituitary – congenital hypopituitarism with multihormonal insufficiency (some known mutations of transcription factors’ genes such as Pit 1, Prop 1, LHX 3, LHX 4, HESX1, but 80-90% of idiopathic origin) or isolated TSH deficiency (TSHB or rTRH genes mutations). The gene discussed is TRH; the disease is cyclic hematopoiesis.