diff --git a/lib/goth/token.ex b/lib/goth/token.ex
index 5356ee5..5323a63 100644
--- a/lib/goth/token.ex
+++ b/lib/goth/token.ex
@@ -42,6 +42,8 @@ defmodule Goth.Token do
 
         * `:metadata` - for fetching token using Google internal metadata service
 
+        * `{:mfa, mfa}` - for building one of the above credentials at runtime
+
       If `:source` is not set, Goth will:
 
         * Check application environment. You can set it with: `config :goth, json: File.read!("credentials.json")`.
@@ -154,6 +156,10 @@ defmodule Goth.Token do
     * `:audience` - the audience you want an identity token for, default to `nil`
       If this parameter is provided, an identity token is returned instead of an access token
 
+  #### MFA tuple - `{:mfa, {mod, fun, args}}`
+
+  For dynamically building a credential source at runtime. This function must return one of the above soruces.
+
   ## Custom HTTP Client
 
   To use a custom HTTP client, define a function that receives a keyword list with fields
@@ -296,6 +302,11 @@ defmodule Goth.Token do
     end
   end
 
+  defp request(%{source: {:mfa, {mod, fun, args}}} = config) do
+    source = apply(mod, fun, args)
+    request(%{config | source: source})
+  end
+
   defp request(%{source: {:service_account, credentials}} = config) do
     request(%{config | source: {:service_account, credentials, []}})
   end