diff --git a/pkg/beacon/metrics_spec.go b/pkg/beacon/metrics_spec.go
index 28095ca..0513cc2 100644
--- a/pkg/beacon/metrics_spec.go
+++ b/pkg/beacon/metrics_spec.go
@@ -327,7 +327,12 @@ func (s *SpecMetrics) observeSpec(ctx context.Context, spec *state.Spec) error {
 	divided := new(big.Int).Div(&spec.TerminalTotalDifficulty, trillion)
 	asFloat, _ := new(big.Float).SetInt(divided).Float64()
 	s.TerminalTotalDifficultyTrillions.Set(asFloat)
-	s.TerminalTotalDifficulty.Set(float64(spec.TerminalTotalDifficulty.Uint64()))
+
+	// big.Int.Uint64 is undefined when the value doesn't fit in 64 bits,
+	// which mainnet's TTD doesn't. Go through big.Float instead so the
+	// gauge reports the real value rather than a silently wrapped one.
+	ttdAsFloat, _ := new(big.Float).SetInt(&spec.TerminalTotalDifficulty).Float64()
+	s.TerminalTotalDifficulty.Set(ttdAsFloat)
 
 	return nil
 }
diff --git a/pkg/beacon/metrics_spec_test.go b/pkg/beacon/metrics_spec_test.go
new file mode 100644
index 0000000..ab76f2b
--- /dev/null
+++ b/pkg/beacon/metrics_spec_test.go
@@ -0,0 +1,51 @@
+package beacon
+
+import (
+	"context"
+	"math/big"
+	"testing"
+
+	"github.com/ethpandaops/beacon/pkg/beacon/state"
+	"github.com/prometheus/client_golang/prometheus"
+	dto "github.com/prometheus/client_model/go"
+	"github.com/sirupsen/logrus"
+)
+
+func gaugeValue(t *testing.T, g prometheus.Gauge) float64 {
+	t.Helper()
+
+	metricDTO := &dto.Metric{}
+	if err := g.Write(metricDTO); err != nil {
+		t.Fatalf("failed to read gauge: %v", err)
+	}
+
+	return metricDTO.Gauge.GetValue()
+}
+
+func TestObserveSpec_TerminalTotalDifficulty(t *testing.T) {
+	prometheus.DefaultRegisterer = prometheus.NewRegistry()
+
+	log := logrus.New()
+	log.SetLevel(logrus.PanicLevel)
+
+	job := NewSpecJob(nil, log, "test_ttd_gauge", map[string]string{})
+
+	mainnetTTD, ok := new(big.Int).SetString("58750000000000000000000", 10)
+	if !ok {
+		t.Fatal("failed to parse mainnet TTD constant")
+	}
+
+	s := &state.Spec{TerminalTotalDifficulty: *mainnetTTD}
+
+	if err := job.observeSpec(context.Background(), s); err != nil {
+		t.Fatalf("observeSpec returned error: %v", err)
+	}
+
+	got := gaugeValue(t, job.TerminalTotalDifficulty)
+
+	want, _ := new(big.Float).SetInt(mainnetTTD).Float64()
+
+	if got != want {
+		t.Fatalf("expected beacon_spec_terminal_total_difficulty to report the real TTD %v, got %v", want, got)
+	}
+}