Who shouldn’t fast? Professional consensus is that people who have eating disorders, who are underweight (a BMI of 18.5 or less*), who are under the age of 18, who are pregnant, who have type 1 diabetes, and who are recovering from surgery should not engage in intermittent fasting. Also, people who are on prescription medications should consult their doctors about their desire to fast. For example, since fasting lowers blood glucose levels, diabetes medications, or any drugs that lower blood glucose levels, might have a greater effect on blood glucose. If you have diabetes treated by blood glucose lowering agents, be aware that you may experience symptoms of hypoglycaemia.

Dom D’agostino mentions the impact of [the ketogenic diet] on Type 1 diabetes 19:16 into this video (take notes and do your research):

Here is an example of a T1D athlete using a ketogenic diet approach:
ketogenicdiabeticathlete.wordpress.com

Another example: livingvertical.org/2015/09/01/the-ketogenic-diet-and-type-1-diabetes-what-i-eat/

Also, I have spoken with several people who are interested in taking advantage of [some form of fasting] but have concerns related to thyroid disorders and related medications. I can only tell them I have found that people [are] able to fast with thyroid conditions, but they are subject to certain limitations. Here is an example:

Tips on how to use intermittent fasting to best effect with thyroid disorders

“Our FastDay community has many people with thyroid problems – whether due to an under-active thyroid, having had their thyroid removed or being on treatment for an over-active thyroid.”

The bottom line: do your research and work with a good doctor. The more you do your research, the more you should be able to determine if you are working with a good doctor.

*Body mass index, or BMI, is the relationship between your weight and your height. A BMI of 20-25 is ideal; 25-30 is overweight and over 30 is obese. If your BMI is under 18.5, you’re considered underweight. If your BMI is 18.5-20, you’re a bit underweight and can’t afford to lose more.