Bone Metabolism in 12-21 Year Olds Undergoing Glucagon Like Peptide (GLP)-1 Receptor Agonist Therapy

Inclusion Criteria: * • Adolescents and young adults with obesity 12-21 years old starting GLP-1 RA therapy (except for dulaglutide or exenatide) or followed with 'usual' care. * Diagnosis of obesity (BMI ≥ 95th percentile for age and sex). The FDA has approved the use of GLP-1 RAs (liraglutide and semaglutide) for adolescents ≥ 12 years old with BMI ≥ 95th percentile for age and sex, and tirzepatide for adults with obesity. Those in the GLP-1 RA arm must have demonstrated efforts at weight loss with 'usual' care, and consistent compliance with appointments and recommendations. * Participants must demonstrate sufficient maturity, psychological stability and cognitive capacity to recognize the significance of being on medical therapy and implement required behavioral changes * Patients taking orlistat as a precursor to GLP-1 RA therapy due to insurance requirements may be included given minimal effects on weight. * Use of the following contraceptive methods is permitted: Combine oral contraceptives (COCs); continuous oral progestin; Progestin-releasing intrauterine device (IUD); Progestin implant; transdermal patch. * Patients with celiac disease will be included if the condition is well controlled and they are on a gluten free diet with normal 25(OH)D levels confirmed by clinical labs within 3 months of enrollment in the study. If a patient does not have recent 25(OH)D results, we will add this to the screening labs. Exclusion Criteria: * • Current or previous history of pregnancy and breast feeding. * Personal or family history of medullary thyroid cancer or multiple endocrine neoplasia type 2 if in the GLP-1 RA group. * \> 5 kg weight loss over 3 months given the known impact of significant weight loss on bone density. * Use of dulaglutide and exenatide (of the GLP-1 RAs) given minimal weight loss with these drugs. * Use of medications such as metformin, phentermine, or topiramate that may cause weight loss, or obesogenic antipsychotic medications if treated for \<3 months, or if dosage is not stable for \>2 months. * Medications other than calcium or vitamin D that affect bone, such as systemic glucocorticoids, phenytoin, phenobarbitone (unless there is a washout period of 3 months prior to enrollment if discontinuation is medically permissible) * Female participants on hormonal contraception will be excluded if this involves use of depot medroxyprogesterone acetate (DMPA). DMPA has profound deleterious effect on bone density, which could confound study outcomes related to bone health. Rationale: DMPA has a well-documented deleterious effect on bone density, which could confound study outcomes related to bone health or metabolic parameters. * Untreated thyroid dysfunction or on stable dose for \<3 months. Primary thyroid dysfunction will be defined as: a TSH ≥ 10 IU/L or low per given reference range with unknown thyroid antibody status, or an abnormal TSH if known positive antibodies. Patients with known hypothyroidism will be included if appropriately treated with levothyroxine and have a normal TSH. For patients with secondary hypothyroidism (deficient production of TSH from the pituitary gland causing hypothyroidism), normal free T4 concentrations (and not TSH alterations) will be used for study inclusion, and recent adjustments in the levothyroxine dose will be permissible as long as free T4 concentrations are in the normal range at dose adjustment (as dose adjustments are often made to get free T4 concentrations in the upper half of the normal range when assessed levels are in the lower half of the normal range). Patients with hyperthyroidism will be excluded given known deleterious effects on both weight and bone metabolism. * Medical conditions known to impact weight or bone density, such as chronic gastrointestinal disorders (including inflammatory bowel disease), other inflammatory conditions, such as rheumatoid arthritis or ankylosing spondylitis, untreated thyroid disease, and hypercortisolemia. * HbA1C \>8% (to avoid deleterious effects on bone from uncontrolled T2DM). * Smoking \>10 cigarettes/day given deleterious effects on bone; substance abuse per DSM-5. * Weight \>450 lbs due to limits for DXA scanners. * History of metabolic and bariatric surgery. * Judged by the investigators to be inappropriate for the study for other reasons not detailed above.