Preheat the oven to 350°F (175°C). Grease and flour a loaf pan.

In a mixing bowl, cream together the softened butter and granulated sugar until light and fluffy.

Add the eggs one at a time, mixing well after each addition.

In another bowl, combine the flour, baking powder, and salt.

Gradually add the dry ingredients to the butter mixture, alternating with the grapefruit juice. Mix until just combined.

Fold in the grapefruit zest.

Pour the batter into the prepared loaf pan and smooth the top.

Bake in the preheated oven for 60 minutes, or until a toothpick inserted into the center comes out clean.

Remove from the oven and let cool in the pan for 10 minutes before transferring to a wire rack.

In a small bowl, whisk together the powdered sugar and grapefruit juice until smooth.

Drizzle the glaze over the cooled pound cake.