underactive thyroid? Any had this and had it treated and then lost weight?

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I suspect I may have an underactive thyroid and am going to ask my doctor about it but just wonderered if anyone out there has ever had this condition and when it was treated, what effect did it have on your weight?

I lost half a stone when I first went on medication for it. Thought fantastic, but then lost no more!!

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SIL has had an underactive thyroid for several years. She put on a couple of stone at first, but managed to lose a stone of it a year ago.