What percentage of patients that have hashimoto's also have lupus?

It's fairly common: While i can't give you an exact percentage, it is common for someone to have more than one auto-immune disease. Lupus, hashimoto's thyroiditis, and sjogren's syndrome are all examples of autoimmune diagnoses that frequently occur together. Even the blood tests can be confusing. A positive ana, the screening test for lupus, also is commonly seen in hashmoto's and sjogren's.

Very small: Hashimoto's is not that uncommon and lupus is much more rare.