Study Design: Following informed consent (and with appropriate subject assent), each subject with TIDM will undergo 5 study visits: Screening, Part A (Sitagliptin 25 mg), Part B (Sitagliptin 50 mg), Part C (Sitagliptin 100 mg), and Part D (Placebo). A randomized double-blinded trial with a 4-period crossover design will be used to establish an ideal Sitagliptin dose for the adult age group (18-30 years). The adult study subject's and the study staff (except one person) will be blinded to the dose of Sitagliptin and they will do all the study Parts A, B, C and D in random order.