The thyroid gland sits in the front of your neck, and secretes a hormone called thyroxine into your bloodstream. This hormone controls the metabolic rate of your body. This is the rate at which all cells in the body work, so that thyroxine controls the tate at which the liver, kidneys, gut and all other organs function.
If the thyroid gland becomes underactive, and secretes too little of the hormone, you feel tiled, cold, constipated, have dry skin and your hair may thin out. This is a common problem in women after the menopause and is controlled by taking the correct dose of thyroxine by tablet on a regular basis.
Your GP is correct, you should take the tablets, and you may well find that you can do far more, and enjoy your life to the full again. You should have regular blood tests to ensure that the dose of thyroid hormone you are taking is adequate.