Because of the remarkable coincidence of the President and First Lady both having Graves' disease, the President's physician is exploring any possible link to environmental or other causes. While the doctors feel it is highly unlikely that their thyroid condition could be related, or in any way related to the lupus disease suffered by Millie, prudence dictates that all such possibilities be examined.
The Secret Service is taking water samples at Camp David, the Vice President's residence, the White House and Walker's Point to ascertain any possible presence of iodine or lithium, two substances which have been associated with thyroid disease. In addition, Dr. Charles L. Christian, head of medicine at the Hospital for Special Surgery in New York, has been asked to review the medical history of the First Family, including Millie, to ensure that there is no relationship in any way. These tests and reviews will be made over the next few weeks. We do not expect conclusions for some time.
George Bush, Statement by Press Secretary Fitzwater on the President's Health Online by Gerhard Peters and John T. Woolley, The American Presidency Project https://www.presidency.ucsb.edu/node/265545