A thesis statement is one or two sentences encompassing the main point of your argument. A good thesis must be one on which people could reasonably disagree. Something to which there is no reasonable counterargument is not a good thesis. Thus, for example, one cannot simply argue that there is a healthcare gap in the United States between Caucasians and minorities, as that is a fact supported by overwhelming evidence. Instead, you need to narrow your topic to a point on which people disagree. As you still seem to be struggling to formulate a thesis, this answer will offer you several possible options for a thesis which you could develop in your paper.
Educational disparities account for a significant part of the healthcare gap. Blacks with a college education have better health outcomes than whites with only a high school education. However, blacks with a college education do worse on several measures of healthcare than whites with a college education. One possible thesis topic for your paper would argue that some other factor, such as racial discrimination, must account for disparities in health outcomes between blacks and white of equal educational and economic backgrounds.
Another possible thesis topic would be that "one size fits all" medical treatments fail women and minorities who may respond differently to treatments than white males. You could develop a thesis arguing for more drug testing to assess differing responses and efficacy based on ethnicity and other genetic factors. You could offer a counterargument that this might constitute unfair racial profiling.