as you mentioned that you are suffering from thyroid problem,hope you must be taking treatment for the same.weight gain and constipation
are symptoms of hypothyroidism
.so if t3/t4 level is below normal or tsh level is high in your blood test
it means taking dose for hypo thyroidism is insufficient.so its better to check for t3/t4 & tsh level, and take a proper dose of thyroxin tablet in the morning.
Important :treatment dose may be needed to increase in winter season. to take vitamin E will be supportive. weight reduction is very impotant .beside exercise low fat ,low carbohydrate
and high protien diet will be helpful.best is to consult endocrinologist