This Certified Reference Material of heavy labeled, intact thyroglobulin (Tg) protein is suitable for use as an internal standard in LC-MS/MS Tg testing methods. Thyroglobulin levels are measured to evaluate effectiveness of treatment for differentiated thyroid carcinoma (DTC) and to monitor for DTC recurrence. Thyroglobulin testing may also be performed to help determine the cause of hyperthyroidism and to monitor the treatment effectiveness of Graves disease.