Yes you describe me to a tea! This is an epic post, I'm sorry, but if someone had told me some of the things I was told at your age id have had a much easier ride over the last 20 years!
I became hypo aged 20 but I suspect I was already a bit, the contraceptive pill made me worse.
The way Janee describes her husbands diet is very much the diet I have always naturally preferred actually. Including the 70-80% dark chocolate!
At 18 I was seven and a half stone, 5'2". You may find at you get a little older that you do 'fill out' more though. My experience being petite and hypothyroid is that small changes can have a big effect. So it's worth becoming very strict about how you take it (don't get too hung up about it though! - remember a missed dose can be taken with the next one). I generally hover between 8 and 8 and a half stone now. When hypothyroid I do loose weight, I think it's actually because I loose muscle.
The amount of thyroxine your body needs is weight related. ive found I often have to change by 12.5 (ie half a 25 tablet); I've done this by alternating 125 one day and 150 the next.
I started on 50 aged 20. I actually didn't need thyroxine when I came off the pill, but then really needed it when I went back on a few years later and then probably lost all my thyroid during my 20s. I probably wasn't very well controlled during my 20s. I spent a year being extremely constipated which ended up being not enought thyroxine. I used to not be brave enough to know that I could ask the gp to double check results.
As you naturally don't hold fat (like me) it's worth keeping a record of Tsh (and t4 / t3 if they do it) and how you feel. Particularly muscle strength.
I'm currently recovering from a brief SLIGHTLY not enough thyroxine phase following the birth of my son. It's wrecked havoc on my muscles and I've been seriously disabled (though I'm waiting to find out if it's anything else). Part of the reason this happened is that I wasn't exercising any more. Hypothyroid myopathy affects the proximal muscles which is what I've experienced.
I think this is the BIGGEST piece of advice I can give to you - I'm 38 - when you feel better, yes eat a really healthy diet (protein and nutrient rich - up to you about gluten etc, personally i feel it's about 'rainbow' foods, nuts, seeds, eggs, protein, fats, etc - following gluten free does actually mean less nutrient empty foods. i love nibbling prunes and almonds and cheese etc during the day) but MOST OF ALL GET STRONG. Get your heart strong through cardio (skipping, swimming etc) but try to improve your muscles, especially upper body, being a woman. Interestingly, I've always enjoyed eating fat and skin from meat. Its probably got muscle and collagen stuff in it!
Sugar and alcohol DO make me put on a little weight though I will say!
I Started building up my upper body by chance aged 26 as I took up capoeira which involves lots of cartwheels and hand stands but it's the lack of this recently that has left my upper body weakened and then strained muscles. (Incidentally I couldn't do them at all at first! Lots of subborn persistence!) But the lack of this has led to really serious issues. I've had to work on expanding my chest and straightening out again. (So swimming is good for this too) Through capoeira I found yoga but specifically a type called forrest yoga which focusses on a lot of physio and core strengthening stuff too - a bit like Pilates. Pilates would be good to do too.
Really, at your age, choose activities which you enjoy, but be aware of lookimg after your muscles. I will add I hated sports etc at your age! I liked hiking, walking, cycling and riding. I found swimming hard as simply no muscles! But, as I say building upper body strenght had helped there.
Because it affects muscles so much, it's why ferritin (iron stores), b12, folate and vit d are important. You especially need iron for growing muscles. Hence a nutrient rich diet. (And dark chocoalte is nutrient rich!) but it might be worth asking your Dr to keep an eye on these levels too, especially if you don't feel great.
Being tiny it may occasionally mean you are on too much thyroxine - I've found this at times, especially when I lost a few pounds and was doing lots of exercise. It's a hard balance. But this means you need to be aware of the risk of osteoporosis (when you're much older) so weight baring exercise such as walking, team sports etc - capoeira!- is wise. Much of your bone density is laid down in the teen years so eating foods which contain calcium is good too (live yoghurt, fish etc) Google katy bowman / katy says alignment matters. She has excellent advice regarding muscles and bones etc.
I am awaking a rheumatology appointment regarding my muscles and , despite gp thinking this is all hypermobility related, I think it's mainly the impact that low iron and low thyroxine has had on me, plus long term stress trying to work and look after a toddler and the house with no strength!
It can take a good few months to fully recover from any myopathy. Abd too much thyroxine can also affect the muscles. Hence why many people would advise t3 aswell - however, I'm personally reluctant at the moment as it has a short half life, and if small changes make a big difference in me I don't want to upset the apple cart. On the other hand, it could be that I really do need the t3 to help. Also, as weight can be an issue I don't want to add t3 if it could affect this.
Before I investigate (battle) for t3 I'm going to see how I am on really good levels of iron, vit d, folate, b12 and regular exercise. I'm already feeling a difference with extra iron tablets. interestingly, I've put on a little weight since upping my iron etc but don't feel well yet. I can literally see and feel muscles returning, though it's an extremely painful process!
(Don't aim to be a body builder - just strong and healthy!)