Patient description: A 16-year-old girl came to the clinic complaining of weight gain and abnormal menstrual cycles. Her BMI was 24, but she has gained 5 kg in the past few weeks. She gets tired more frequently and does not have energy to go dancing with her friends. Her lab results were remarkable for high TSH levels (15 mU/L) and low free T4 levels (0.18 ng/dl). Her anti-TPO levels were extremely high (136 IU/ml). She was diagnosed with Hashimoto disease. She does not smoke and she is not sexually active.

Clinical trial eligibility criteria: eligible ages (years): 10.0-17.0,  Weight Gain Ages 10 to 17 years (inclusive) Receiving treatment with ziprasidone, aripiprazole or clozapine for the past 56 days or longer Clinically stable on current treatment regimen (see Rationales below) Stable dose of current psychotropic co-medications for at least 30 days BMI increase of > 7% within 3 months OR a > 0.5 BMI z-score increase within the past 24 months while taking olanzapine, quetiapine or risperidone, with maintenance of the threshold level of weight gain Lifetime diagnosis of a schizophrenia spectrum disorder (schizophrenia, schizoaffective disorder, psychotic disorder NOS), a bipolar spectrum disorder (bipolar disorder, bipolar disorder NOS), certain mood disorders (mood disorder NOS, major depressive disorder with psychotic features), or certain disruptive/aggressive disorders (conduct disorder, intermittent explosive disorder, autism spectrum disorder with history of clinically significant levels of disruptive behaviors as defined below) using DSM-IV determined by clinical interview and K-SADS-PL Sexually active girls must agree to use an effective form of birth control or be abstinent Principle caretaker is able to participate in study appointments as is clinically indicated guardian and the child must agree (legally consent and assent) to participation  Any medication that would significantly alter glucose, insulin or lipid levels. Prohibited medications will but are not limited to: insulin, steroids, topiramate, sibutramine, orlistat, metformin, amantadine, vitamin E (other than in standard multivitamins), antidiabetic drugs, HIV drugs Major neurological disorder or medical illness that affects weight gain (e.g., unstable thyroid disease), requires a prohibited systemic medication or procedure (e.g., diabetes mellitus [insulin], chronic renal failure [steroids]) or that would prevent participation in physical activity in the healthy lifestyle program Current active thyroid (TSH >18 microIU/ml), hepatic (2 LFTs >4x upper limits of normal), renal (serum Creatinine >1.4 mg/dL in females and serum Creatinine >1.5 mg/dL in males), cardiac, gastrointestinal, or adrenal disease Fasting glucose > 125 mg/dL on two occasions indicating need for prompt treatment for diabetes Child meets DSM-IV for substance abuse or dependence disorder within the past month, not including tobacco abuse or dependence • Current treatment with more than one antipsychotic medication Current treatment with more than 5 total psychotropic medications (i.e., 4 psychotropics plus SGA) Known hypersensitivity to metformin Pregnant or breast feeding Current or lifetime diagnosis of anorexia nervosa or bulimia nervosa Significant risk for dangerousness to self or to others that makes participating inadvisable