diff --git a/CHANGELOG.md b/CHANGELOG.md
index 843aadea..597e2288 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+
+- Replace Alertmanager RoundTripper with custom NewRequest.
+
 ## [0.12.0] - 2024-11-05
 
 ### Added
diff --git a/pkg/alertmanager/alertmanager.go b/pkg/alertmanager/alertmanager.go
index bf255d40..206e6b4c 100644
--- a/pkg/alertmanager/alertmanager.go
+++ b/pkg/alertmanager/alertmanager.go
@@ -18,8 +18,9 @@ type Config struct {
 }
 
 type AlertManager struct {
-	address string
-	client  *http.Client
+	address  string
+	tenantId string
+	client   *http.Client
 }
 
 func New(config Config) (*AlertManager, error) {
@@ -27,15 +28,10 @@ func New(config Config) (*AlertManager, error) {
 		return nil, microerror.Maskf(invalidConfigError, "%T.Address must not be empty", config)
 	}
 
-	client := http.Client{
-		Transport: &customTransport{
-			tenantId: config.TenantId,
-		},
-	}
-
 	return &AlertManager{
-		address: config.Address,
-		client:  &client,
+		address:  config.Address,
+		client:   http.DefaultClient,
+		tenantId: config.TenantId,
 	}, nil
 }
 
@@ -62,7 +58,13 @@ func (am *AlertManager) CreateSilence(s *Silence) error {
 		return microerror.Mask(err)
 	}
 
-	resp, err := am.client.Post(endpoint, "application/json", bytes.NewBuffer(jsonValues))
+	req, err := am.NewRequest(http.MethodPost, endpoint, bytes.NewBuffer(jsonValues))
+	if err != nil {
+		return microerror.Mask(err)
+	}
+	req.Header.Add("Content-Type", "application/json")
+
+	resp, err := am.client.Do(req)
 	if err != nil {
 		return microerror.Mask(err)
 	}
@@ -101,7 +103,12 @@ func (am *AlertManager) ListSilences() ([]Silence, error) {
 
 	var silences []Silence
 
-	resp, err := am.client.Get(endpoint)
+	req, err := am.NewRequest(http.MethodGet, endpoint, nil)
+	if err != nil {
+		return nil, microerror.Mask(err)
+	}
+
+	resp, err := am.client.Do(req)
 	if err != nil {
 		return nil, microerror.Mask(err)
 	}
@@ -132,7 +139,7 @@ func (am *AlertManager) ListSilences() ([]Silence, error) {
 func (am *AlertManager) DeleteSilenceByID(id string) error {
 	endpoint := fmt.Sprintf("%s/api/v2/silence/%s", am.address, id)
 
-	req, err := http.NewRequest("DELETE", endpoint, nil)
+	req, err := am.NewRequest(http.MethodDelete, endpoint, nil)
 	if err != nil {
 		return microerror.Mask(err)
 	}
diff --git a/pkg/alertmanager/http.go b/pkg/alertmanager/http.go
new file mode 100644
index 00000000..0ea03c5b
--- /dev/null
+++ b/pkg/alertmanager/http.go
@@ -0,0 +1,21 @@
+package alertmanager
+
+import (
+	"io"
+	"net/http"
+)
+
+// NewRequest creates a new http.Request with the given method, url and body.
+// It adds the tenantId as X-Scope-OrgID header to the request if it is set.
+func (am *AlertManager) NewRequest(method, url string, body io.Reader) (*http.Request, error) {
+	req, err := http.NewRequest(method, url, body)
+	if err != nil {
+		return nil, err
+	}
+
+	if am.tenantId != "" {
+		req.Header.Add("X-Scope-OrgID", am.tenantId)
+	}
+
+	return req, nil
+}
diff --git a/pkg/alertmanager/transport.go b/pkg/alertmanager/transport.go
deleted file mode 100644
index 94b19f41..00000000
--- a/pkg/alertmanager/transport.go
+++ /dev/null
@@ -1,14 +0,0 @@
-package alertmanager
-
-import "net/http"
-
-type customTransport struct {
-	tenantId string
-}
-
-func (t *customTransport) RoundTrip(req *http.Request) (*http.Response, error) {
-	if t.tenantId != "" {
-		req.Header.Add("X-Scope-OrgID", t.tenantId)
-	}
-	return http.DefaultTransport.RoundTrip(req)
-}