In a handful of examples, I've noticed that the endomorphism ring $\mathrm{End}(R,+,0)$ is isomorphic to the ring $R$ itself. For instance, $\mathrm{End}(\mathbb{Z},+,0)\cong\mathbb{Z}$ and $\mathrm{End}(\mathbb{Z}/n\mathbb{Z},+,0)\cong\mathbb{Z}/n\mathbb{Z}$.
Is this true in general, or are there examples of rings which are not isomorphic to the endomorphism ring as above? If not, is it at least always true for $R$ a field? Thanks.