People with an under-active thyroid tend to have a low base metabolic rate. This is a leading cause of weight gain as the body can’t burn fat quickly enough and the excess is stored. This can also cause dieters to gain weight as their metabolism drops when not eating as much.
It tends to be more common in women for a number of reasons. The main one is that women spend much of their lives dieting to control their weight. This undermines your metabolism and causes it to drop which is a compounding factor for under-active thyroids, especially during the menopause.
Another reason it affects mainly women is that women tend to internalise stress, which causes a lot of internal conflict. This stress causes adrenal, brain and thyroid issues. Another key factor affecting mostly women is that women experience monthly hormone fluctuations, some of which do permanent damage to thyroid function.
This weight gain is difficult to control as dieting itself affects the thyroid by way of the drop in. metabolic rate. However you can chose to speed up your metabolism through exercise and a healthy, balanced diet.