We advised a particular client to eat more lamb kidneys given his particular state of health. He said he had heard that it is makruh or haram.  We disagreed. 

“It says in al-Mudawwanah: that which may be included with meat, such as fat, liver, stomach, heart, lungs, spleen, kidneys, neck, testicles, feet, head and the like, comes under the same ruling as meat.[i.e. it is halal]” 

Tahdheeb al-Mudawwanah by al-Baraadha‘i (1/93). See also Mawaahib al-Jaleel (6/204)