Hi there..
thyroid is endocrine gland which secretes thyroxine hormone which is essential for growth, metabolism and other functions of body.. patient could have hypothyroidism or hyperthyroidism..
Hypothyroidism means that there is less synthesis of thyroxine hormone( due to iodine deficiency, pituitary causes, infection or autoimmune disease of thyroid gland) which leads to weight gain, weakness, easy fatiguablilty, loss of appetite.... treatment of which is thyroxine supplements in form of tablets..
Hyperthyroidism is condition where there is excess synthesis of thyroxine( due to pituitary causes, tumors of thyroid gland..) leading to weight loss,increased appetite, excess sweating, palpitations, heart problems..there will be enlargement of thyroid gland(swelling in front of neck) which could compress on voice box, wind pipe, food pipe, leading to respiratory difficulty, voice change, swallowing difficulty..
Symptoms of various diseases overlap so don't worry as its not cancer..