Underactive thyroid, or hypothyroidism is a relatively common condition. It is more common in women then men, and more often shows up in middle age.
The symptoms are tiredness, weakness, weight gain/difficulty losing weight, lowered libido, irregular periods and some people may get a swelling in the neck (around the throat).
It's pretty easily tested for, so if you are worried, then see your GP for a simple blood test. If you do have it, it can be easily treated with replacement of the thyroid hormone. This is one of the best medications you can take because it is basically same as the normal thyroid hormone that you aren't making enough of, so you can't be allergic to it, and there are no side effects aside from if you take too much of it.
You'd need semi-regular blood tests for a little while just to get the dosage right, but otherwise people tend to live normal, healthy lives, with most people remaining on thyroid hormone tablets for life.
Seeing your GP for a test is a good idea. If you do have the condition, your GP can give you some more information.
There may be some people on the forum who suffer from it, so they can offer their viewpoint - but generally, once treated, the symptoms disappear and you should be right to lose weight normally.