Sounds a lot like my symptoms I had when my thyroid started going hypo (after years of leaning more towards hyper). Has she been tested for Hashimoto's? If your doctor refuses to check her thyroid you probably want to go get a second opinion. Make sure they check TSH, FT4, FT4, and TG and TPO antibodies. Doctors often refuse to check the FTs and antibodies, but they're extremely important. I would try to find a doctor that would be willing to do a full thyroid panel that includes all the above. Thyroid issues are no joke, but some doctors don't take them as seriously as they should or fail to adequately test for them.
I developed a similar rash on my chest. It was really itchy and disgusting. It appeared around the time my thyroid started going hypo (less than a year before my neck started swelling) and never went away on its own (I had to go to a dermatologist to have it removed with liquid nitrogen). My doctors kept insisting that it wasn't related to my thyroid but after reading your post I'm pretty sure it was. Thank you. I always wondered why I had it.
What's the reference range for the Free T4? Ranges vary from lab to lab and have to come from your own report(s). With a TSH of only 0.81, we'd expect Free T4 to be higher than it is.
I'm guessing there isn't a Free T3 test? If so, what was the result with reference range? Free T3 is the hormone the body (individual cells) actually use, whereas Free T4 is a storage hormone that must be converted to Free T3 before it's used.
That said, with the low TSH and low FT4, Central (or Secondary) hypothyroidism could be the problem vs Hashimoto's. With Central hypothyroidism, there's a problem with the pituitary gland not producing enough TSH to adequately stimulate the thyroid, whereas Hashimoto's causes the body to produce antibodies that destroy the thyroid.
The ultrasound will tell quite a bit, but you need to insist on the full thyroid panel (Free T4, Free T3, TSH and thyroid antibodies as noted above). Also, if your daughter has been tested for Vitamin B-12 and D, please post those results as well. I'd also recommend Ferritin, as well, which is the iron storage hormone. B-12 and D deficiencies are common with those of us with hypothyroidism. B-12 deficiency also produces a type of anemia that can be mistaken for iron deficiency (though Sarah is right that eating ice is a sign of iron deficiency). B-12 deficiency can also be missed if one concentrates only on the iron issue, even though the 2 deficiencies can co-exist. B-12 deficiency also produces extreme fatigue/exhaustion, brain fog, easy bruising, and other symptoms of hypothyroidism.
Both Vitamin B-12 and D are necessary for production/metabolism of thyroid hormones. Iron is necessary for the conversion of Free T4 to Free T3.
I had an enlarged thyroid and lots of menstrual symptoms. My thyroid had lots of nodules and ultimately ended up needing to be removed because of those
nodules, but I was much older (37) at the time, and had been having symptoms for years, so hopefully your daughter just has an enlarged thyroid. (Even if she has some nodules, it is still unlikely that any of those nodules are cancerous or that she will need surgery.)
Have they tested her for Hashimoto's? Hashimoto's is an autoimmune disease where the immune system attacks the thyroid. This can cause hypothyroidism and an enlarged thyroid. Prolonged and very heavy periods can be a symptom of hypothyroidism (one that I had and have lots of experience with). The combination of hypothyroidism and anemia can exacerbate symptoms since both of those things can cause fatigue, brain fog, and lots of problems. Because Hashimoto's can cause the thyroid to grow and make more thyroid hormone before the immune attack continues and destroys more thyroid tissue, thyroid hormone blood tests might not always show there is a problem.
Chewing ice is a symptom of anemia. I use to chew ice at night for years! I finally stopped once I started taking an iron supplement every day (instead of a few times a month, which is what I had been doing when I had a serious ice addiction) - I was still anemic though when I stopped.
I am not sure if the rash is a symptom of hypothyroidism or anemia (could be, dry rough skin is). I do know that a lot of autoimmune diseases can cause rashes on the skin, and that is someone has one autoimmune disease they are more likely to have others.
If you haven't already been to the appointment, I would strongly ask for her to be tested for Hashimoto's. It is a blood test (that involves a frozen blood sample, I had to do mine at a blood draw center and not my PCP) where they test for two antibodies, anti-thyroglobulin and anti-thyroid peroxidase. Doctors seem to be reluctant, but with the weight gain and menstrual irregularites and rash, I would push for it just to rule it out. If it isn't Hashi's, then you could look for other causes, but I would look there (if it were me).
I gained 35-40 pounds in 2015 and 2016, had very heavy periods, and when I noticed lumps on my neck in 2018, my TSH was 3.14 (range 0.45 to I think 5), and was told I was fine, despite clearly having what was physically and mentally crippling hypothyroidism (which much later I've learned was combined with anemia which made everything much worse for me). I had Hashimoto's, and probably had for years. I started having seasonal allergies all the time in 2006, was diagnosed with a thyroid disorder in 2018, but I'm pretty sure my Hashi's started before my seasonal allergies and is part of the reason I had such a very strong response to any sort of pollen or mold spores that come my way.
If you haven't already, I would strongly push to get her menstrual irregularities evaluated by a gynecologist, now, even before you figure out whether they are caused by hypothyroidism or not. I thought mine would just get better once I no longer had hypothyroidism, but months of being severely hypo (or years of high estrogen caused by Hashi's and hypothyroidism) caused me to have a very large fibroid which caused an intense amount of blood loss at the the end of 2019 and beginning of 2020 (thought I would need to go to the ER the amount of blood loss and size of clots was horrendous - no one should experience that). It was impossible for me to get to the right thyroid hormone until we stopped the constant monthly blood loss. (I don't think your daughter has a fibroid, but I wish I had the cause of my blood loss evaluated sooner in hindsight - I assumed it was just hypo related and probably was at the beginning, but months of being very hypo seems to have caused a big estrogen imbalance for me.)
All I can say is I'm so sorry your daughter is experiencing this. Now that I am fully on the other side, no more (or very minor) hypothyroidism, and after 2 fibroid surgeries and months of estrogen suppressing drugs in 2020, I'm no longer anemic, no longer experiencing heavy or prolonged periods. It was hell when I was going through it, but I made it to the other side and I hope your daughter will too!
Please feel free to ask about the ultrasound results once you have them... I had thyroid cancer, but again, I had undiagnosed Hashimoto's and hypothyroidism for many, many years and I don't think my situation is that common. I know it can be scary, especially if there are nodules, but more thyroid nodules are not cancerous, and my situation isn't that common for people with Hashimoto's.
It is frustrating, you are looking for quick answers but the pieces of the puzzle.need to be assembled first. The initial blood test on thyroid levels is only step one. The ultrasound should rule out or confirm any suspect growth where then it could lead to a biopsy. Have they checked for certain thyroid antibodies that could gauge overall thyroid health? Make sure you are seeing an endocrinologist. Be patient as they go down the list and let's hope it is nothing serious.