My son (who is 18) was diagnosed with an underactive thyroid yesterday. His TSH was 7.71, he was prescribed Thyroxine 50mg. However I am not convinced, he is very UNDER WEIGHT BMI is very low and confirms underweight. This was the reason I took him to the Drs. If anything I thought he may have had an overactive thyroid. He eats absolutely loads and although he is at Uni, eats exceptionally healthy. I spoke to my regular GP today and voiced my concerns, he agreed with me that something isn't quite right and being so slim just does not match his TSH, he wants to do more tests and has said do not give Thyroxine yet. I am so worried, any ideas or opinions please?