Had gastric bypass 15 years ago and now have sudden (3 month) 30lb weight gain - what is the cause?

The gastric bypass lasts approximately 10 years, and since you do not have a thyroid it is more difficult to lose weight. I recommend that you try to follow a keto diet, do intermittent fasting, and exercise daily, this for 6 months, you will not see. a quick change but it will be the beginning of a healing process

Several factors could be contributing to the sudden weight gain you're experiencing, especially considering your history with gastric bypass surgery, thyroid cancer, and autoimmune conditions.
Your thyroid function could be a significant factor in the weight gain. Having no thyroid can make it difficult to regulate metabolism, and fluctuations in thyroid hormone levels could be contributing to the weight changes you're noticing. If your thyroid levels aren't stable, this can slow down your metabolism and make it harder to lose or maintain weight, even with the same diet and exercise. It might be worth having your doctor assess whether adjustments to your thyroid medication are needed, or if additional testing is required to ensure proper thyroid function.
The medications you're taking, particularly Xeljanz for lupus, PSA, and RA, as well as antidepressants or anti-anxiety medications, could also be playing a role. Weight gain is a known side effect of several of these medications. It would be beneficial to review your current medications with your healthcare provider to determine if any could be contributing to the weight gain and if there are alternatives that may have fewer side effects.
Your autoimmune conditions could also be influencing your metabolism. Lupus, rheumatoid arthritis, and other related conditions can affect your body's ability to regulate weight, especially when combined with the medications you are taking. These conditions may lead to hormonal imbalances that contribute to weight fluctuations.
Although you mentioned that your diet and activity levels have remained unchanged, it’s possible that small, unnoticeable changes in eating habits, stress, or lifestyle might be impacting your weight. Stress, anxiety, or depression can often lead to emotional eating or cravings, which may contribute to weight gain over time.
Lastly, while gastric bypass surgery is effective for many, it is important to recognize that some patients experience weight regain after several years. This can happen due to changes in stomach anatomy, hormonal changes, or shifts in eating habits. If the weight gain continues despite your efforts, it may be worth consulting with a specialist to evaluate how your gastric bypass has held up over time.
To move forward, I recommend scheduling an appointment with your healthcare provider to address these factors comprehensively. They may suggest additional testing to assess your thyroid function, review your medications for any side effects, and evaluate other potential causes like hormonal imbalances. It’s also important to consider your mental health and its impact on your weight, as stress and emotional well-being can play a big role in managing weight long term. A tailored approach will help you navigate the challenges you’re facing and guide you toward a solution.

Hello, I hope you are well. It should be noted that the effective time period for a gastric bypass is 5 years. A 30 pound weight gain is not concerning based on the time period and the amount of time that has passed since your surgery. My recommendation would be to follow up with your team's nutritionists and exercise regime before any more weight is gained.