diff --git a/Doc/c-api/init.rst b/Doc/c-api/init.rst
--- a/Doc/c-api/init.rst
+++ b/Doc/c-api/init.rst
@@ -646,6 +646,15 @@
    :c:func:`PyGILState_Release` on the same thread.
 
 
+.. c:function:: PyThreadState PyGILState_GetThisThreadState()
+
+   Get the current thread state for this thread.  May return ``NULL`` if no
+   GILState API has been used on the current thread.  Note the main thread
+   always has such a thread-state, even if no auto-thread-state call has been
+   made on the main thread. The purpose of this function is mainly a
+   helper/diagnostic tool.
+
+
 The following macros are normally used without a trailing semicolon; look for
 example usage in the Python source distribution.