I used to never have to worry about what I ate as a kid or teenager, seemed to have a naturally high metabolism, was pretty much always underweight or low end of normal range weight. Then at 18ish developed anorexia for various reasons. This totally messed with my metabolism and digestive system. My weight went up and done over the years So that even eating a normal amount of food for average woman in her 20s for my height I ended up at nearly 9st (126lbs) at one point may not seem a high weight but was for me and is at high end of healthy weight for my height. I then lost weight again though still ok BMI. Finally got decent counselling and was good BMI and slim for about 5 years.
Suddenly over a month, even though I was eating normally with at least one take away a week, no counting kcals etc. I lost about a stone (14/15lbs) went to Gp as I did not want to be underweight or ill.
Got diagnosed with Graves Disease which is an autoimmune disease causing hyperthyroidism and can be very dangerous if left untreated. I was very tachycardia resting heartbeat was 140bpm, low blood pressure, hand tremors, felt really warm even when room cold, severe night sweats etc
Luckily medication worked for me after about 18-24 months.
I was then fine for a couple years after. Then annual bloods showed I was now hypothyroid. Only symptom was tiredness really. Basically autoimmune disease totally destroyed my thyroid. Been on eltroxin for this for about 6/7 years.
Since this I very much have to watch what I eat or put on weight, I'm sure my age doesn't help either (early 40s)! I'm a healthy weight now but definitely can't eat whatever I want and not gain weight.
I'm pretty active and do sport at least 4 days a week. I have to be very mindful of calorie intake these days or weight seems to go on very easily.
Nobody in my life would really know that I now have to work at being slim, I have to make good food choices if I don't want to gain weight.
I've no desire to be underweight anymore but do want to stay at a healthy weight. If I put on weight it also goes on my tummy which isn't healthy either so I am trying to avoid that!