well i do know a little about thyroid, i had an under active or hypo for two years then i got thyroid cancer. it is very unlikely that your thyroid is causing the weight, unless of course it is truly under active to the point you need synthriod, you would have other symptoms though,always cold,always tired to name a couple. you should see your doctor for a blood test to check the thyroid levels. be careful about trying to change the level on your own...the thyroid does control every system in the body including kidneys which could be damaged by too high levels.all that being said, any food containing iodine affects thyroid there is also a supplement called thyroid support you can purchase at any health food, vitamin shop. proceed with caution, even when my thyroid levels were extremely low, i never put on weight. thyroid has to be very very low for a prolonged time for this to happen.
A person with a thyroid problem should eat more vegetables, leaner meats, more beans and fiber rich foods. Always use table salt that has potassium iodate and avoid iodine supplements unless recommended by a doctor.