diff --git a/synapse/rest/client/sync.py b/synapse/rest/client/sync.py
index cc1edb2ca3..cfb662c9a2 100644
--- a/synapse/rest/client/sync.py
+++ b/synapse/rest/client/sync.py
@@ -1142,10 +1142,9 @@ def _serialise_threads(
     if threads.updates:
         out["updates"] = {
             room_id: {
-                thread_root_id: {
-                    "thread_root": update.thread_root,
-                    "prev_batch": update.prev_batch,
-                }
+                thread_root_id: attr.asdict(
+                    update, filter=lambda _attr, v: v is not None
+                )
                 for thread_root_id, update in thread_updates.items()
             }
             for room_id, thread_updates in threads.updates.items()
diff --git a/synapse/types/handlers/sliding_sync.py b/synapse/types/handlers/sliding_sync.py
index c9d0f32b1f..01679f4fb0 100644
--- a/synapse/types/handlers/sliding_sync.py
+++ b/synapse/types/handlers/sliding_sync.py
@@ -412,6 +412,9 @@ class SlidingSyncResult:
                 # TODO: comment
                 prev_batch: Optional[StreamToken]
 
+                def __bool__(self) -> bool:
+                    return bool(self.thread_root) or bool(self.prev_batch)
+
             updates: Optional[Mapping[str, Mapping[str, ThreadUpdate]]]
             prev_batch: Optional[int]
 
diff --git a/tests/rest/client/sliding_sync/test_extension_threads.py b/tests/rest/client/sliding_sync/test_extension_threads.py
index ffe47f68d4..4e350ceac5 100644
--- a/tests/rest/client/sliding_sync/test_extension_threads.py
+++ b/tests/rest/client/sliding_sync/test_extension_threads.py
@@ -153,16 +153,7 @@ class SlidingSyncThreadsExtensionTestCase(SlidingSyncBase):
         # Assert
         self.assertEqual(
             response_body["extensions"][EXT_NAME],
-            {
-                "updates": {
-                    room_id: {
-                        thread_root_id: {
-                            "thread_root": None,
-                            "prev_batch": None,
-                        }
-                    }
-                }
-            },
+            {"updates": {room_id: {thread_root_id: {}}}},
         )
 
     def test_threads_incremental_sync(self) -> None:
@@ -214,14 +205,5 @@ class SlidingSyncThreadsExtensionTestCase(SlidingSyncBase):
         # Assert
         self.assertEqual(
             response_body["extensions"][EXT_NAME],
-            {
-                "updates": {
-                    room_id: {
-                        thread_root_id: {
-                            "thread_root": None,
-                            "prev_batch": None,
-                        }
-                    }
-                }
-            },
+            {"updates": {room_id: {thread_root_id: {}}}},
         )