I am not a lawyer (and I don't play one on TV!), but I can not imagine that there is a "universal" law that would cover all hospitals across the country as it would seem more appropriate that each entity would want to set their own policy.
A quick Wiki search on "hospital accreditation" shows that there are no less than 6 organizations in the United States that make some attempt at standardizing the health care industry. The article goes on to sayObviously there are no standards for providing health care from one hospital to the next, so why would there be some provision that would say whether or not hospitals in general would allow you to carry your weapon?"The different accreditation schemes vary in quality, size, intent and the skill of their marketing. They also vary considerable in terms of the cost incurred by hospitals and healthcare institutions. They have varying degrees of commitment to assessing medical ethical standards and clinical standards."
I hope this helps.