diff --git a/synapse/storage/registration.py b/synapse/storage/registration.py
index e404fa72d..a6aa64f9f 100644
--- a/synapse/storage/registration.py
+++ b/synapse/storage/registration.py
@@ -458,17 +458,27 @@ class RegistrationStore(background_updates.BackgroundUpdateStore):
 
     @defer.inlineCallbacks
     def get_user_id_by_threepid(self, medium, address):
-        ret = yield self._simple_select_one(
-            "user_threepids",
-            {
-                "medium": medium,
-                "address": address
-            },
-            ['user_id'], True, 'get_user_id_by_threepid'
+        def f(txn):
+            sql = (
+                "SELECT user_id"
+                " FROM user_threepids"
+                " WHERE medium = ? AND LOWER(address) = LOWER(?)"
+            )
+            txn.execute(sql, (medium, address))
+            row = txn.fetchone()
+            if not row:
+                return None
+            if txn.rowcount > 1:
+                raise StoreError(500, "More than one row matched")
+            return {
+                "user_id": row[0]
+            }
+
+        res = yield self.runInteraction(
+            "get_user_id_by_threepid", f
         )
-        if ret:
-            defer.returnValue(ret['user_id'])
-        defer.returnValue(None)
+
+        defer.returnValue(res)
 
     def user_delete_threepids(self, user_id):
         return self._simple_delete(
diff --git a/synapse/storage/schema/delta/36/user_threepids_medium_address_index.sql b/synapse/storage/schema/delta/36/user_threepids_medium_address_index.sql
new file mode 100644
index 000000000..702a87278
--- /dev/null
+++ b/synapse/storage/schema/delta/36/user_threepids_medium_address_index.sql
@@ -0,0 +1,16 @@
+/* Copyright 2016 OpenMarket Ltd
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+CREATE INDEX user_threepids_medium_address on user_threepids (medium, LOWER(address));