diff --git a/fastapi/routing.py b/fastapi/routing.py
index 49f1b6013..a78bfc0bc 100644
--- a/fastapi/routing.py
+++ b/fastapi/routing.py
@@ -20,6 +20,9 @@ from typing import (
     Type,
     Union,
 )
+import anyio
+from anyio import CapacityLimiter
+import functools
 
 from fastapi import params
 from fastapi._compat import (
@@ -163,9 +166,10 @@ async def serialize_response(
         if is_coroutine:
             value, errors_ = field.validate(response_content, {}, loc=("response",))
         else:
-            value, errors_ = await run_in_threadpool(
-                field.validate, response_content, {}, loc=("response",)
-            )
+            # Run without a capacity limit for similar reasons as marked in fastapi/concurrency.py
+            exit_limiter = CapacityLimiter(1)
+            validate_func = functools.partial(field.validate, loc=("response",))
+            value, errors_ = await anyio.to_thread.run_sync(validate_func, response_content, {}, limiter=exit_limiter)
         if isinstance(errors_, list):
             errors.extend(errors_)
         elif errors_: