Hypothyroidism issues. Functionality?

Hi, My mother diagnosed with hyperthyroidism  5 years back and on medicine Neo-Mercazole 10 for one year then doctor suggested    Neo-Mercazole 5 again after one year Doctor found Hypothyroidism and suggested  Eltroxin 50 and same is still my mother continuing as suggested by doctor. Every 4-5 months we do check Thyroid profile of her and that always coming normal for last 3 years. But for last 1 year she is increasing weight and gain 16 kgs where as she is vegetarian with less oil and only whole masala in cooking and dat also in minimum quantity. She does yoga and  walk for 2 hrs daily also cook for herself and other household work. Her lipid profile, CBC, liver kidney profile, sugar, vit D, Vit 12 al reports are normal. Please suggest do I need any further investigation on thyroid ?? She is 65 years old. Need suggestions from Endocrinologist what more investigation should my mother go through to get full clear picture of her thyroid gland functionality ?