Impact of Vitamin D Therapy on Thyroid Function and Antibody Levels in Pediatric Graves' Disease

Inclusion Criteria: * All new pediatric participants aged 9-17 years with a new diagnosis of GD who will be started on methimazole, will be offered to participate at the time of diagnosis. * Biochemical features include: * Suppressed TSH \<0.1. * Elevated T3 * Elevated Free T4 * Elevated T4 * Positive TSI or TRAb. The presence of antibodies is diagnostic. * Our study will offer enrollment to non-English speaking participants Exclusion Criteria: * Initial hydroxy vitamin D levels \>80 ng/mL * Hypocalcemia, corrected calcium based on albumin \<8.4 mg/dL * Hypercalcemia, corrected calcium based on albumin \>10.5 mg/dL * Conditions that affect vitamin D metabolism such as: malabsorption, chronic kidney or liver disease, nephrocalcinosis, hyperparathyroidism * Current use of medications which are known to affect thyroid function or vitamin D metabolism such as thyroid hormone replacement, corticosteroids, anticonvulsants * Allergy to vitamin D or methimazole * Diagnosis of Hashitoxicosis or thyrotoxicosis (both TSH receptor antibody (TRAb) and thyroid-stimulating immunoglobulin (TSI) levels are negative) * Participants under the age of 9 years at the time of diagnosis * Pregnant participants * Active or uncontrolled infections, other significant medical conditions deemed by the investigator to interfere with study participation or pose undue risk to the participant.