First, which Rehmannia ? There is Shu di Huang/processed Rehmannia and Sheng di Huang/Unprocessed Rehmannia. Shu Di is mostly often used as a tonic and I know of no evidence that either is CI with Graves disease. The unprocessed Rehmannia is the stronger anti-inflammatory and used for autoimmune disease. As to which herbs you pair it with depends on the person you are treating, it can be combined with Baikal Skullcap, Chai Hu, Self Heal, Zhi Mu and many other herbs depending on the specifics of the case