How do doctors rule out other causes of weight gain (thyroid issues, PCOS, medication side effects) before recommending GLP-1?

Generally yes. A proper weight-management workup usually includes screening for common underlying contributors before (or alongside) considering medication. That typically means thyroid function bloodwork (an underactive thyroid can contribute to weight gain), a review of current medications (some antidepressants, steroids, and other drugs are known to cause weight gain as a side effect), and for women, a look at symptoms that might suggest PCOS. It's not meant to be a hunt for an excuse, more about making sure the treatment plan actually targets what's going on. References: https://obesitycanada.ca/healthcare-professionals/adult-clinical-practice-guideline/

Found out my weight gain was partly a medication side effect from something I'd been on for years, doctor caught it during the review before even discussing GLP-1 options.