I got to thinking about this, and I think it's very important that you get your medical problems taken care of immediately. If you don't protect your health, you will be unable to protect your children.
If I am understanding correctly, you have a hyper-active thyroid that is not completely under control with medication and a lump that has not been biopsied. As I see it, lack of money should not be a reason to postpone taking care of these medical issues. Especially in these times, doctors and hospitals are willing to extend very reasonable payment plans. Even if you end up in bankruptcy because of medical bills, your children will be better off with you alive and healthy.
Hyper-thyroidism that is not controlled can undermine your health and make any other illness harder to get over. The lump may just be a cyst, but the mental stress of wondering about it adds to the other unavoidable stresses you have had to endure. If it's worse than a cyst, early treatment is much more successful than late treatment.
Consider who is going to get custody of your children if something happens to you! To protect your children, you really need to take care of your health issues now.