Hypothyroidism, swimming, and hypothermia

This is something that has just recently become an issue. I have mild hypothyroidism (high TSH). I am on some treatment for it, but I'm not at the level that the doctor wants me at yet. My normal body temperature is 97, and sometimes lower, and my resting heart rate is around 48 (60-80 is average). I've recently started doing some light swimming at night to soothe my joints from my Lyme disease. The temperature of the water is around 80 degrees. I know that you can get hypothermia from being in water this temperature, but I thought that it took a long time. I stay in the water for about an hour, but I find myself getting very cold after only about ten minutes. This does not seem normal. I have to keep swimming. Otherwise, I start to shiver and get really cold. I don't think that I should be getting cold this quickly, and I wonder if my low thyroid and low metabolism is to blame. Has anyone else experienced anything like this?

Thanks. I figured that it was most likely my hypothyroidism causing the problem. I am supposed to start Synthroid next weekend. I'll see if that helps. If not, I'll just have to figure out a way to keep moving or get out of the pool periodically.