FASTING - NEED ADVICE

I have trouble loosing weight as I have an underactive thyroid and although I can loose weight it is a very very slow process. Anyway, been to see the nurse today for some advice about another matter and thought I would mention it to see if she had any advice and she said I am obviously eating too much, which at the moment have been eating around 1400 a day but doing quite a bit of exercise to. Ok, so I should lower my calorie intake, which is fine. (I am 42, 5'6" female and 221lbs) The issue I have is that she told me to fast for 3 days a week and only drink water and the other 4 days to just eat healthy!!! Well I cannot believe that she was suggesting this and asked how long for and she said until I loose at least 28lbs. I do not believe this is a healthy way to loose weight, I would like other peoples opinion please