Objective: Pediatrician is the first contact in the Pathway to Care in children with Mental retardation (MR). Following the recent advancements in the area of molecular genetics, understanding of specific conditions of MR or Developmental Delay (DD) is expanding. Hypothyroidism is a treatable metabolic/endocrinological cause of MR. The aim of this study was to determine the prevalence of hypothyroidism in school children with mental retardation. Objective: Pediatrician is the first contact in the Pathway to Care in children with Mental retardation (MR). Following the recent advancements in the area of molecular genetics, understanding of specific conditions of MR or Developmental Delay (DD) is expanding. Hypothyroidism is a treatable metabolic/endocrinological cause of MR. The aim of this study was to determine the prevalence of hypothyroidism in school children with mental retardation. Material & Methods: The study was conducted with 140 randomly selected school-going children with MR residing at the Government Institute of Mentally Retarded Children, Chandigarh. Thyroid Stimulating Hormone (TSH) levels were estimated to find out the prevalence of hypothyroidism. Results: The prevalence of hypothyroidism in children with MR was 10.7% with mean TSH levels of 12.08 [plus or minus] 4.58 mIU/ml. Majority of the children with Down syndrome had hypothyroidism. Developmental delay was found in all the children. Mean IQ levels were significantly lower in children with MR and hypothyroidism when compared with euthyroid children with MR. Significant negative correlation was present between IQ and TSH levels in the MR children with hypothyroidism Conclusion: The results of our study suggest that hypothyroidism may be a causative or exaggerating factor of MR in these children. Hence, mass screening of this treatable endocrine disorder in newborns as well as children diagnosed with MR is strongly recommended for identification and intervention of developmental delay in the community. (Contains 1 figure and 4 tables.)