The death of the 15-year-old mare named Lady was being investigated by the city’s health department, officials said.

The 15-year-old Standardbred cross horse died due to a small tumor in her adrenal gland that likely caused an aortic rupture, a necropsy found, a TWU representative said.

The cause of death of a carriage horse that collapsed and died in Hell's Kitchen Tuesday has been determined, the union representing carriage horse operators said Friday.  Lady, a 15-year-old horse that had just been brought to New York City from Pennsylvania in June,