The most important scriptural principle to remember here is contentment. We must be content with what we have. We must be thankful for what God has given us. This is an overarching principle that covers all aspects of life - 1 Timothy 6:6, Proverbs 14:30, Philippians 4:11, just to name a few. Contentment is a basic, fundamental character trait that we must learn.
Proverbs 5:18 applies this principle to marriage - be content with your wife, rejoice in her, enjoy her. And if you are content with her, and find ways to enjoy her, then you will not be tempted to infidelity. On the other hand, if you do NOT find contentment with her, you will be greatly tempted by other women. This can be dealt with positively, by marrying someone else, but can alternatively lead to sin. It is therefore dangerous to direct your sexual fantasies towards a hypothetical second woman, because it could cause you to jump into bed with the wrong one too rapidly (even the best men are susceptible to sexual temptation). Direct your sexual fantasies back to your wife, and you will then be able to calmly and carefully consider a second marriage with less risk of jumping into it to satisfy unsatisfied lust.
I have known Sarah for over 15 years, most of which we have been married. Over that time, her weight has fluctuated greatly for various reasons, including health ones - I can sympathise with your wife having thyroid problems. The entire time, I have found her very attractive. Do I have preferences in her weight? Yes, because I want her to be healthy. But I am fundamentally attracted to her, as a person. Her personality causes me to be attracted to her body. In fact, I was attracted to her personality before I was attracted to her physically. It is the fact that she is Sarah that makes her attractive to me. Physical form is just the icing on the cake (and I like the icing too, but the cake tastes great even if the icing is imperfect).
You need to learn to enjoy what you have. While overweight, she'll have awesome breasts. While slim, she'll have an awesome waist. Overweight there's padding over the bones (cushioning in intimacy), slim she's easy to carry. There's something to enjoy in every situation.
Obviously, health is important.
@Daniel DeLuca, the point others are making is that you are looking at the superficial final symptom of poor health (obesity), and trying to attack that symptom (with drugs). Others are suggesting that a more holistic view of health would be far more beneficial - hence suggestions about removing sugar from your diets etc. These are positive, loving suggestions from people who have been where you are and have worked through it. They are trying to help you learn from their experiences, and their mistakes. Everything is being said in love - but you are struggling to perceive it that way for emotional reasons. The more you struggle to perceive it, the more strongly people speak, culminating in hyperbole like "do you want to kill her" being used to emphasise an opinion that what you are doing is unhealthy. Such statements are simply an attempt to get you to listen to what is being said. They are not an accusation of actual evil intent.
Calm down. If you need to leave the conversation to do that, then leave the conversation until you're calm.
Maybe pull off your wife's clothes and find some detail that you can really, really rejoice in for a while...