The current ATA (American Thyroid Association) and Endocrine Society position is that routine thyroid screening (ultrasound or calcitonin) is NOT recommended before or during GLP-1 RA therapy in patients without clinical suspicion or family history of MTC[5].

Routine calcitonin screening would actually cause harm due to false positives. Basal calcitonin has poor specificity for MTC — it can be elevated by proton pump inhibitors, renal impairment, smoking, chronic thyroiditis, and other conditions. A false positive calcitonin would lead to unnecessary thyroidectomy with associated surgical risks (recurrent laryngeal nerve injury, hypoparathyroidism).

When to screen:

• Personal history of MTC — contraindication to GLP-1 RA (do not prescribe)
• Family history of MTC or MEN2 — contraindication (genetic testing for RET proto-oncogene mutations)
• Palpable thyroid nodule discovered incidentally — standard thyroid nodule workup per ATA guidelines
• Symptoms suggestive of MTC (rapidly growing neck mass, new diarrhea in specific clinical context) — calcitonin + thyroid ultrasound

Bottom line: ask about family history of MTC/MEN2 before prescribing. Do not order routine thyroid screening in low-risk patients.

Thank you all — this is incredibly reassuring. I have no family history of thyroid cancer and no thyroid symptoms. Based on what you've explained, the risk seems to be essentially theoretical. I feel much more comfortable starting the medication.

That's the right conclusion. And I'd emphasize that your doctor told you not to worry about it — that's consistent with how essentially every endocrinologist, internist, and obesity medicine specialist interprets the current evidence.

The broader lesson is about how to read FDA warnings. Boxed warnings don't always mean "this drug causes this problem in humans." They sometimes mean "this problem was observed in preclinical testing and we can't absolutely prove it doesn't apply to humans." The level of concern should be calibrated to the actual evidence, not to the font size on the label.

That said, the precautionary approach of asking about MTC family history before prescribing is good practice. It's a simple screening question that takes 10 seconds and addresses the one scenario where the theoretical concern could be clinically meaningful.

Well-discussed thread. Summary: The thyroid C-cell concern with GLP-1 RAs is based on rodent findings that do not translate to humans due to fundamental differences in C-cell GLP-1R expression between species. Over 50,000 patient-years of clinical trial data show no MTC signal. The risk-benefit ratio overwhelmingly favors treatment. Routine thyroid screening is not indicated, but family history of MTC/MEN2 should be assessed before prescribing.