lower metabolism?

Curious if they say how long before this happens. I'm 5 feet even, would love to lose 15+ pounds I've gained due to hypothyroidism... I started eating VERY low calories 3 days ago, only 900 plus exercise and just added fast-5 today. I know it's way under what's recommended but I'm so short that at 1200 calories I was not moving the scale whatsoever. Definitely will be testing the starvation mode theory.

I'm hoping to gradually move up calories throwing in one high calorie day a week as I've read on some other forums. Some suggest taking a break and going back to IF.

I've been obsessed with reading about IF since I discovered it a few days ago and I noticed that a lot of people are getting results without restricting calories as long as they are an adequate amount of calories, which is why they say that you may not experience weight loss first few weeks and then it starts at 1 pound per week. Obviously if you are over eating and you lose weight then your new weight by nature will require fewer calories to maintain. In my case I am eating way way less then what my maintenance level will be at my goal weight, so slowing my metabolism even further is a bit scary.

This was one of the things I did make a study off when began an IF.

The metabolism does not get lower - most of the energy your body needs is for your basal metabolic rate. (bmr)

Let's say your bmr is around 2000 cal. That is what your body needs just for being you.
If you then do some running and burn 200 cal your total for that day would be 2200 cal. If you hadn't had that run you would still burn the 2000 cal - every day year round.