Just wanted to chime in with some encouragement/advice. I know how it feels to REALLY want to have a baby and yet see your weight as being an obstacle. It stinks.
I was 275 when I got pregnant and my pregnancy ended 8 weeks early with rapid onset preeclampsia and HELLP syndrome. My care team agrees that obesity was a factor.
You already have blood pressure issues and it sounds like you are thinking that there may be a thyroid issue as well. Those things, coupled with obesity, make you at risk for preeclampsia.
That being said, we are not living in the dark ages and you won't be on your own to deal with any potential complications. Your OB will know your risk factors and monitor you accordingly. If things start looking weird during your pregnancy, there are supportive care options to keep you going as long as possible.
My advice would be to see about your thyroid and then give yourself a few months to get your weight down. Even if you only lose a small amount of weight, your body will thank you for it during your pregnancy. I'd also caution you not to set goals like, "I'll allow myself to get pregnant when I reach 140 pounds." It's just not realistic. Get to a point where you feel positive, then get busy!