Preheat the oven to 325 degrees F. Spray a 9" x 5" loaf pan with vegetable oil spray. Line the sides and bottom of the pan with parchment paper.
Combine the flour and baking powder in a bowl. Stir in the pistachios. Place the butter and sugar in a mixing bowl and beat until lightened. Beat in the eggs, one at a time. Mix in the zests and juices. Add the flour mixture into the wet ingredients and mix until combined. Spoon the batter into the pan.
Bake for 1 hour and 40 minutes, or until the loaf springs back when touched in the centre. Let the loaf cool on a baking rack for 30 minutes. Unmould and cool to room temperature before slicing and serving.
Note: Shelled, unsalted pistachios are sold in the bulk foods department.