Thyroid treatment was used at the beginning of the 20th century for people with Down's syndrome, and an improvement in the physical and mental response was noted (Smith & Berg, 1976). Later studies reported conflicting findings about the prevalence and type of thyroid dysfunction (Sare et al, 1978; Lobo et al, 1980) but there now seems little doubt about the existence of a high prevalence of hypothyroidism (Mani, 1988; Dinani & Carpenter, 1990).