When it comes to weight gain, nothing beats the thought that thyroid is the first culprit. It is a well-known fact that overactive thyroid can drive weight loss and underactive thyroid can drive weight gain. When you have an underactive thyroid, the metabolic rate of the body slows down. Further due to inadequate thyroid hormone in the body, the person tends to add more water to his body. If low thyroid persists for a long time, weight gain can go on.
But the weight that can be attributed to thyroid is in the range of 2-5 kg. This extra fluid may disappear once the thyroid function is brought to normal. However since hypothyroidism develops slowly, most patients don’t lose substantial weight even after thyroid is normalized.
Sometimes thyroxine in higher doses is used to achieve weight loss. However this is not advisable as high levels of thyroxine may cause fast heart beats (palpitation) and bone loss (osteoporosis) especially in older persons.
To sum up, underactive thyroid contributes partly to weight gain, but is not the sole reason. This weight attributed to underactive thyroid can be lost once thyroid is normalized with thyroxine.