I would call a malfunctioning thyroid a serious problem whether over active or under active. I would hesitate to generalize about the weight of some one since the thyroid is part of the adrenal system and all the systems of the body can malfunction in numerous ways.
We need to find more symptoms like weight gain, cancer, diabetes, high blood pressure, nervousness. You an not simply look at one symptom.
May I point something out? Just because a doctor can name a disease does not mean he can heal it. In fact doctors can not heal anything. We pay doctors for diagnoses.
A person suffering from a symptom needs to correct the cause in order to heal. Plus very rarely is it that there will be just one symptom.
Let me give a simple example: A doctor will test your blood for sugar. If there is a certain amount in the blood he will proclaim you to be diabetic. At a lesser amount he will tell you that you are not diabetic. But if you have elevated insulin in the blood we would know that you are at risk of type II diabetes and cancer. Perhaps it would be more useful if the doctor measured the amount of insulin in the blood which is rarely done.
How often does a doctor check the patient for nutritional deficiencies? Not very often. Why not? The reasons doctors do not check for vitamin and mineral deficiencies is because prescribing drugs and procedures are required. Doctors are severly disciplined if they do not adhere to accepted medical protocols. The use of vitamins and minerals is not an acceptable medical practice.
I am not saying that going to a doctor is a bad idea. I am saying that you do need to recognize the limitations of doctoring and how their mission is to treat illness not heal the body. You can not get the right solutions by asking the wrong questions.