RET and medullary thyroid gland carcinoma: This psychometric analysis generated evidence to support the use of the Functional Assessment of Cancer Therapy item GP5 (GP5) as a measure of tolerability and confirms the appropriateness of categorizing “high side-effect burden” using a rating of 3 or 4 (score ranges 0–4) in patients with advanced/metastatic RET-mutant medullary thyroid cancer (MTC).