The basic enrollment requirement to maintain F-1 status is that you be full-time enrolled during normal semesters and make normal progress toward your degree. But sometimes, it is not possible for you to complete the minimum number of credits required or even within the typical 8 semesters due to circumstances beyond your control. The regulations allow you to be enrolled in fewer than 3 credits for specific academic situations or for medical reasons.
In all circumstances requiring you to reduce your course load or not register for any classes, it is imperative that you discuss the matter with the ISA before taking action. The federal regulations provide relief when the ISA is able to take timely action in advance. The regulations do not permit retroactive authorizations.
Of course, in all matters concerning your classes, you will speak with Student Affairs, the Dean of the College, the Registrar, your Academic Advisor, etc. in addition to the ISA.
Any authorized reduced course loads for either academic or medical reasons, are legitimate reasons to extend the dates of your program on your I-20. However, it is your responsibility to request (from the ISA) any necessary extension.