diff --git a/include/mbedtls/threading.h b/include/mbedtls/threading.h
index ed16a23..c136ea0 100644
--- a/include/mbedtls/threading.h
+++ b/include/mbedtls/threading.h
@@ -28,10 +28,13 @@
 #include <pthread.h>
 typedef struct mbedtls_threading_mutex_t {
     pthread_mutex_t MBEDTLS_PRIVATE(mutex);
-    /* is_valid is 0 after a failed init or a free, and nonzero after a
-     * successful init. This field is not considered part of the public
-     * API of Mbed TLS and may change without notice. */
+
+    /* is_valid is controlled by code in tests/src/threading_helpers - it will
+     * be 0 after a failed init or a free, and nonzero after a successful init.
+     * This field is for testing only and thus not considered part of the
+     * public API of Mbed TLS and may change without notice. */
     char MBEDTLS_PRIVATE(is_valid);
+
 } mbedtls_threading_mutex_t;
 #endif