Hypothyroidism (also called low thyroid or underactive thyroid) is one of the main medical causes of weight gain or failure to lose weight on a diet. If you're dieting but gaining weight, or if you find that standard weight loss diets do not work for you, then you should consider whether you may have a sluggish thyroid, especially if you have other hypothyroid symptoms too such as tiredness, lethargy, feeling cold, dry and pale skin, menstrual problems, female infertility, poor memory or inability to concentrate.
The incidence of low thyroid is increasing, and it is actually very common, especially in women, and after middle age. Unfortunately it is very often a missed diagnosis, because doctors are unaware how common it is and do not look for it. It is also frequently underdiagnosed because the 'normal' range for thyroid blood test results is too wide, according to many thyroid experts.
Even if it is diagnosed, and you are being treated with thyroxine, this doesn't mean that you are receiving optimal treatment, or that your thyroid function has been brought to normal levels. For this reason, you could be suffering from inability to lose weight or difficulty in losing weight due to an underactive thyroid, despite being on treatment for hypothyroidism.
More information on how low thyroid and other factors can prevent successful weight loss and what to do about it is explained in the book "Why Can't I Lose Weight".