Thyroid deficiency (hypothyroidism) can cause a number of symptoms including weight gain, low energy levels, constipation, dry skin, sensitivity to cold, depression, joint stiffness, and hair loss. Low thyroid function is very common, particularly in women, and becomes more common with advancing age.
While it is true that thyroid hormones influence the body's basal metabolic rate (BMR) - the rate at which we burn calories - they're not the only factor involved. Many other (non-thyroid) hormones play a role, as do assorted proteins and body chemicals, all of which influence energy intake (the amount of calories you consume), energy expenditure (the number of calories you burn) and, by extension, body weight. Correcting a thyroid deficiency could result in some weight loss - or perhaps simply prevent weight gain- but taking supplemental thyroid hormone isn't a magic bullet for maintaining energy balance. Because this is such a complex process, there's no way to predict the effect on weight of treatment for low thyroid function. And even if a thyroid deficiency is appropriately corrected, you can still gain weight if you take in more calories than you burn.
Don't depend on thyroid medication for weight control. Instead, I suggest that you talk to your health care practitioner about your weight concerns, discuss what aspects of your lifestyle might contribute to your weight gain and request reliable information and actionable advice about diet and exercise that could help you stop gaining and start losing.
Andrew Weil, M.D.