Calcitonin Blood Test
A blood test measuring calcitonin levels, used primarily as a tumour marker for medullary thyroid carcinoma (MTC). Calcitonin monitoring may be recommended during GLP-1 receptor agonist therapy due to the theoretical C-cell tumour risk. Elevated calcitonin levels warrant further thyroid investigation.
Technical Context
Serum calcitonin measurement: by immunometric assay (chemiluminescent or electrochemiluminescent). Reference ranges: typically <10 pg/mL for women and <17 pg/mL for men (assay-specific). Elevated calcitonin: medullary thyroid carcinoma (MTC — calcitonin >100 pg/mL is highly suspicious; >500 pg/mL is virtually diagnostic), C-cell hyperplasia (MTC precursor — moderately elevated), renal failure (reduced clearance), proton pump inhibitors (mild elevation from gastrin-mediated C-cell stimulation), and various non-thyroid cancers (rare). Regarding GLP-1 RA therapy: routine calcitonin screening before or during GLP-1 RA therapy is NOT recommended by clinical guidelines (ADA, Endocrine Society) because: MTC is very rare (~0.01%), calcitonin screening has low positive predictive value (many false positives causing unnecessary workups), and the C-cell tumour signal is from rodent studies with uncertain human relevance. However, if calcitonin is incidentally elevated, thyroid ultrasound and endocrine referral are warranted.