Dieting and Hypothyroid medication

Did you just start taking thyroid medication? When I first started I had rapid weight loss. The thyroid's function is to regulate metabolism. Do not stop taking it or cut back yourself. If you are worried you should see your doctor and have a thyroid panel checked. Even after being on it you will need periodic checks to make sure you are taking the correct amount. When I first started mine, my doctor check my TSH, T3, T4, free T4 every few months, now he just checks it every 6 months or if I'm feeling bad-feeling cold in the middle of summer, tired even after sleeping well, feeling like I can't get enough rest...
Hope that helped. Keep up the good work!

My aunt has experienced similar rapid weight loss since going on her thyroid medication a few months ago. She goes to the gym most days and watches what she eats just like you and she lost a large amount of weight in the first few months and now that she has less to lose (is closer to her goal), the weight loss has slowed down. I think it partly depends on how much weight you have to lose total.

What I'm wondering is - since you weigh almost exactly what I weigh and I'm struggling and not losing anything...how many calories a day are you consuming? I, too, am exercising at least 6 days a week -- walking, lifting weights, and playing tennis with some basketball and bicycling mixed in -- and I can't seem to lose anything although I'm exercising around 8 hours a week and watching what I eat very carefully.

What sort of calorie level is working for you?

Were you eating 1200 calories and exercising this much for a long time or have you just started that recently (when you started losing weight rapidly)?

I guess with hypothyroidism conventional wisdom doesn't mean much but generally speaking (aside from the hypothyroidism), someone of your weight and activity level should have a maintenance calorie level closer to 2100-2500 a day which means if you've been eating closer to 1200 a day rapid weight loss would definitely be expected. In fact, you could lose a pound every two or three days on average depending on how many calories you burned exercising along with your calorie deficit in your diet.

Since you've been on the medication for a long time, the sudden change in weight is curious. If you were always eating and exercising like this but just suddenly started losing weight then I would say that is very odd. If you just started losing weight rapidly when you started eating this low calorie level and exercising this much, then it would seem your metabolism may be working normally. If that is the case though, you may want to consider upping your calories a little to avoid slowing your metabolism in the future. From many books on diet and metabolism that I've read, they say you shouldn't go below your resting metabolic rate or you risk slowing down your metabolism. Your RMR is based on gender, age, weight and activity level so I can't quote it exactly but it is probably around 1800 calories a day for you. There are free online calculators for it if you are curious.