Your thyroid hormones are actually really important. They do too many things to reply here, but the actions pertinent to weight are metabolism, protein synthesis, and efficient use of your macros and micros. They also keep you warm, therefore my comment, I've been wearing a wool sweater anywhere the AC is set lower than 78. My metabolism is definitely slower. I'm 5 months pregnant and do about 1700 calories a day and am gaining weight the last 2 months faster than I would like to, 14 lbs (total this pregnancy) vs the 10 I was trying to calculate on MFP. My maintainence is usually about 1400/1500. Luckily, it's the easiest fix ever. A daily pill and you're totally fine. Now that I know what's going on I can adjust my diet a little too. It's hard to know what's real weight gain and what's bloating when you are pregnant, but now that I'm aware of the problem I can fix it. Hope this was the answer you were looking for!