Preheat the oven to 350°F and prepare a loaf pan with butter or non-stick spray. TIP: I used the wrapper from my butter and it worked perfectly!
In a medium mixing bowl, mix together the flour, baking soda, and baking powder and set it aside. Zest the grapefruit and then juice it. Strain the pulp from the liquid, discard the pulp, and set the juice and zest aside as well.
Place the butter and sugar in the bowl of a stand mixer (or a large mixing bowl if using a hand mixer) and beat together until creamy, about 5-6 minutes. Add the eggs, grapefruit juice, and zest and continue beating until everything is combined. Remove the bowl from the stand mixer.
Add half of the dry mixture and half of the milk to the batter and mix in with a silicone spatula or spoon. Once combined, add the rest of the dry ingredients and the rest of the milk and fully incorporate everything.
Pour the batter into the prepared loaf pan, tap lightly on the counter to remove any air bubbles, and bake for 42-45 minutes or when a toothpick comes out clean. Remove the pan from the oven and allow the loaf to cool in the pan for 10 minutes and then on a cooling rack.
Once the loaf has cooled, combine the powdered sugar and grapefruit juice in a medium bowl and whisk together until smooth. Immediately pour over the top of the cooled loaf and allow the glaze to set. Sprinkle with more fresh zest if desired and enjoy!