You did not respond to my questions, but I want to impress on you te importance of thyroid on metabolism. There is information saying that hypothyroid patients can have Resting Metabolic Rate (RMR) that is 200 - 900 calories per day too low. If diet and exercise remain unchanged and your RMR drops by just 200 calories per day, you would gain about 46 pounds over time.
What makes you think your thyroid is low functioning? Have you been tested for the actual thyroid hormones, Free T4 and Free T3? If so post the results and reference ranges shown on the lab report.
Even more important than lab tests is an evaluation for symptoms. Please look at this link and tell us which ones you seem to have.
https://www.mayoclinic.org/diseases-conditions/hypothyroidism/symptoms-causes/syc-20350284