“Things in themselves” and noumena are similar in Kantian metaphysics (Critique of Pure Reason, mostly) and interchangeable much of the time. The phenomena/noumena divide is integral to Kantian metaphysics, so it is natural to wonder where “things in themselves” fall.
The Stanford Encyclopedia of Philosophy has a useful section discussing this (Things in themselves, noumena, and the transcendental object) where noumena are divided into “noumena in a positive sense” (positive noumena) and “noumena in a negative sense” (negative noumena). Then it is argued “things in themselves” are a subclass of negative noumena.
However, this provokes the question “What is the difference between positive noumena and negative noumena?”. The entry is difficult to understand on this matter.
Other natural questions are “Are there negative noumena that are not things in themselves? If so, what are they? What are some examples?”.