What tests have been done to determine Graves Disease? Thyroid Stimulating Immunoglobulin (TSI) is the definitive test for Graves.
Please post results, with reference ranges, so we can be more familiar with his situation.
Sorry I didn't make that more abundantly clear
"It should be pointed out that, especially in the US literature, the term ‘hashitoxicosis’ is sometimes used to describe an autoimmune thyroid disease overlap syndrome of Graves’ and Hashimoto’s disease. In this article the term is strictly limited to the ‘leakage’ symptoms of active Hashimoto’s disease." - Hashitoxicosis – Three Cases and a Review of the Literature
The recommended treatment for the Graves/Hashimoto's type of Hashitoxicosis is block and replacement therapy (anti thyroid medication as well as thyroid medication). Surgery is a last resort if symptoms cannot be controlled with this type of therapy.
Hashitoxicosis is a hyper phase of Hashimoto's Thyroiditis. Does your fiance actually have hyperthyroidism?
Hashimoto's Thyroiditis is an autoimmune thyroid disorder, in which the body sees the thyroid as foreign and produces antibodies to destroy it. It's typically, associated with hypothyroidism, but can be characterized by periods of hyperthyroidism.
Euthyrox is a T4 (levothyroxine sodium) medication that's used to treat hypothyroidism, which is under active thyroid. It would not be prescribed for someone who has hyperthyroidism, which is over active thyroid.
Please post whatever thyroid related blood tests your fiance has had done. They should have done Free T3 and Free T4, which are actual thyroid hormones; Thyroid Stimulating Hormone (TSH); and antibody tests, Thyroid Peroxidase Antibodies (TPOab) and Thyroglobulin Antibodies (TGab). The Free T3 and Free T4 levels would determine whether your fiance is hyper or hypo. High levels would indicate hyper, low levels would indicate hypo. TSH is a pituitary hormone produced to stimulate the thyroid. When one is hyper, it's, typically, very low and when one is hypo, it's, typically, higher than normal.
Once you post those results, we will be able to better assess the situation and give you some better answers. Be sure to post the lab's reference ranges from the report as ranges vary lab to lab and have to come from his own report.