I gained a bunch of weight when I was developing hypothyroidism even though my calorie intake and activity level didn't change.

125mcg levothyroxine and 5mcg liothyronine were the meds/doses that resulted in my levels being optimized back in June 2011.

I have needed small dose adjustments since then to keep my levels optimized and currently take alternating doses of 125/137mcg levothyroxine and 10mcg liothyronine.

All of the weight I gained slowly came off in the past 2+ years since being optimized.

I did not diet - I just continued eating an appropriate number of calories while making healthy choices.... and I continued exercising as I always have (5-6 days/week of cardio/strength training....and always taking the stairs/parking far from entrances, etc. :).

My TSH was 122. Within 6 weeks of starting on 88mcg of Levoxyl I lost 12 lbs. I will have to say that I got so used to eating hardly anything to maintain my weight that that's probably what took it off.
I am up 6lbs since then, but decided to give myself a break. I am tall and within my weight range for my height.
My drugs have changed...I then had 20mcg of Cytomel added, and then switched to (105mg) of Armour (current).
I may be close to being optimized. I still have a hair issue. I have few new follicles, but symptoms will lag behind drugs.
Crossing my fingers....

I have always been skinny..I know that's why being hypo was never investigated, in my case, by the docs I saw..I have lost even more now since starting on Armour in June..
Before that I was taking Synthroid.
Let's say since I have been taking levothyroxine these last 3 yrs..I have lost about 4-5lbs..on a 99 lb frame it's quite a loss..

I have always been normal weight for my height so as sovgrateful said, that probably delayed my diagnosis because I always appear very healthy. When I started adequate doses of T3 and T4, I dropped 10 lbs very quickly, 10 lbs I did not want to lose. I have been on a high calorie diet to get the weight back in line (avocados, organic coconut oil, lots of fruit in the morning, (no sugars) and my weight has pretty much leveled out back to normal. Also hit the gym 3X a week for adequate muscle mass and toning, all important to well being.

I have been on levothyroxine for almost 15 years. Occasionally the dosage is changed. When I first started this med, I lost 60 pounds in about 3 months. Now my weight goes up and down. I am still overweight, but I think that is just because I like to eat. Good luck finding the right meds and dosages for you problem. I was lucky and got the right meds the first time. I do notice that if I forget to take my meds then by mid afternoon I am exhausted. It keeps me on my toes to take meds regularly as prescribed.