I haven't taken my Synthroid (thyroxine) in about a month. Since that month I have gained 25lbs. Is it because I'm off my synthroid (thyroxine)?

Probably two reasons. If you have been prescribed synthroid or levothyroxine replacement therapy, then your body likely needs thyroid hormone therapy to maintain normal thyroid levels. When you stop synthroid, and the thyroid levels become very low, you can then gain weight from two major factors: fluid retention, and slowed metabolism. Please check with your doctor to discuss what dose of synthroid you should restart.
Watch your diet. Not necesserly. Watch your carbs. Stop drinking any pop.
Most likely. Depending on your synthroid (thyroxine) dosage and degree of the lack of your own bodys thyroid hormones. Coming off synthroid (thyroxine) abruptly can cause some serious disturbances in your heart, metabolism, and effect other hormones of your body. You must be feeling sluggish. Not sure why you did, I'm sure you have a good reason for it but if you dont like synthroid (thyroxine)! you have other options.