If the tumor is malignant, the malignancy could explain the weight loss. I wouldn't be inclined to treat with chemotherapy either. Has she had a CBC done? How are red blood cells? White blood cells? Hemoglobin? The blood tests could be wrong, it's not uncommon with hyper/hypothyroidism. Maybe talk to your vet about treating and see if there is any improvement??
Roxy - Feb 2005 - 75 lb American Staffordshire Terrier (adopted Jul 2011)
Peewee - Jan 2006 - 8 lb Chihuahua (adopted May 2009)
Myka - Nov 1998 to Jan 2010 - 85 lb American Pit Bull Terrier cross
Lacy - Sept 1992 to July 2003 - 18 lb Reg Shetland Sheepdog