What is the ruling on a suitor informing the woman he proposes to that he has Hashimoto's disease and the potential for developing diabetes as a result? And what is the ruling on believing doctors when they report the possibility of an illness?

If the aforementioned illness does not cause aversion or contagion, then there is no obligation to inform the prospective fiancée about it, as it is among the defects that do not necessitate annulment. The words of trustworthy doctors are to be considered, but one should be wary of obsessive thoughts and rely upon Allah, for He is the one who benefits and harms. This is in accordance with His, the Almighty's, saying: ﴿And if Allah should touch you with adversity, there is no remover of it except Him; and if He should intend for you some good, then there is no repeller of His bounty. He causes it to reach whom He wills of His servants. And He is the Forgiving, the Merciful﴾ [Quran 10:107]. And in accordance with His, peace be upon him, saying: "Know that if the entire nation were to gather to benefit you with something, they would not benefit you except with something Allah has already written for you. And if they were to gather to harm you with something, they would not harm you except with something Allah has already written for you. The pens have been lifted, and the pages have dried."