Gaining Weight

My doctor put me on eltroxin about 6 months ago for a under active thyroid and I gained 3.8 kg in the last 4 months. Its not that much but I weighed 49-50 kg for the last 6 years and never gained weight easily. My eating habits and exercise program didn't change but my weight suddenly started going up. I don't drink my eltroxin regularly like prescribed. I sometimes forget to take it for 2-3 days and take it again for the next 5 and then not for a day or 2. Can the fact that I take the eltroxin so irregular be the reason for my weight gain?