Current guidelines vary on how large a nodule needs to be to do a biopsy, but usually they have to be at least 1cm no matter what suspicious features are present. It is not clear from your description if the nodule is 5.1 mm in the biggest dimension or if it is 5.1 mm taller than wide, but if the largest dimension is 5.1 mm then they will monitor it by ultrasound to see if it gets bigger or not. I can't tell you "not to freak out", but most thyroid nodules are very slow growing and most are benign, and you are already doing what you need to to monitor yours. If your nodule is already bigger than 1.5 cm, they will probably do a biopsy, but again, most thyroid nodules are benign.
I'm going to keep the thyroid nodules and hypo symptoms as two different things because nodules and being hypo aren't necessarily related (you can have nodules and no other thyroid issues, or have hypothyroidism or hyperthyroidism and no nodules).
As for elevated levels of thyroglobulin (Tg) and your hypothyroidism symptoms - it sounds like you probably have Hashimoto's. Did they also test your thyroid peroxidase antibodies (TPO)?
Hashimoto's is an auto-immune disease where you develop antibodies that attack your thyroid, which can cause a decrease in thyroid hormone (hypothyroidism) and stimulate an increase in TSH. Your TSH might come back "normal" despite having a lot of hypo symptoms, as early stages of Hashimoto's can cause your TSH to fluctuate and sometimes you might even have hyperthyroidism symptoms. If you do have Hashimoto's, there is a whole lot of information on the internet about diet changes you can make to try to decrease your thyroid antibodies and slow down progression of Hashimoto's disease, although I am not sure how many people are helped by changing their diets (and I discovered my Hashimoto's too late to try this myself). You may also need to take thyroid hormone to help supplement what your thyroid isn't producing, and that may help with your hypothyroidism symptoms and help you to feel better.
Are they also testing for freeT4 and freeT3 levels? T4 is produced by the thyroid and T4 is converted to the active form of T3 throughout the body and both freeT4 and freeT3 tell a lot more about your thyroid hormone function than TSH. If freeT3 and freeT4 are "in range" but in the lower parts of their ranges, you might have hypo symptoms.
Should you be worried? I think you are doing what you need to do to figure out what is going on with your thyroid nodules and also what is causing your hypothyroidism symptoms. Lots of people have nodules and don't know about it (for women, my endocrinologist told me 20% of 20 year olds, 30% of 30 year olds, 40% of 40 year olds, etc. have thyroid nodules). It is very common and most of these nodules are benign, and now that you know you have one, you are monitoring it which is good.
Understanding and treating your Hashimoto's or hypothyroidism (if that is what you have) is a good idea because Hashimoto's is a progressive disease and may continue to get worse as the thyroid struggles to keep up with demand for hormone.
I hope this helps - I went through a nodule diagnosis and Hashimoto's diagnosis last year, and spent the first few months "freaking out" myself, so I know how stressful it is to not know what is going on.
ATA/AACE guidelines now recommend that biopsy be done at 2 cm unless certain criteria are met. Having a solid nodule would be one criteria, but it appears that your nodule is "soft" (not solid?), so that's possibly why it's recommended that biopsy be performed at 1.5 cm.
I agree that it's being monitored as much as can be and most nodules aren't cancer (< 5% are cancer), so my advice would be to "not" freak out, since that's counter-productive in any case.
I also agree that you should be having Free T4 and Free T3 tested along with the TSH as T4 and T3 are the actual thyroid hormones whereas TSH is a pituitary hormone, not a thyroid hormone. The body does produce mostly T4, but also some T3, with the remainder of T3 being converted from T4. The Free T4 and Free T3 are those amounts of each of the hormones that are not bound by proteins and are readily available for use - Free T4 to be converted to Free T3 and Free T3 to be used by individual cells.
Do you know what your actual Thyroglobulin Antibody (TgAb) level was? Elevated TgAb indicates that you have Hashimoto's, which as Sarajogs mentioned is an autoimmune condition that attacks the thyroid. Hashimoto's can be present for many years before actual thyroid hormone levels go out of range, but can cause symptoms. Your symptoms are those often seen with hypothyroidism.
There's a lot of controversy over the diet issue and whether or not it can help Hashimoto's... Some say diet can cure, or reverse, Hashimoto's - so far, the science doesn't bear that out, however, changing diet to eliminate foods that cause inflammation, weight gain or that one might be sensitive to (even if we don't realize it) is always beneficial no matter how advanced our Hashimoto's might be. For many of us, the main foods that cause inflammation include things that contain gluten (wheat, barley and rye), sugar, dairy and of course, since soy is a goitrogen, I always recommend eliminating that. An elimination diet might be helpful.
It would also be helpful if you could get your doctor to test your Vitamin B-12, D and Ferritin. Ferritin is the iron storage hormone and iron is necessary for the conversion of Free T4 to Free T3. Vitamins B-12 and D are necessary for proper metabolism of thyroid hormones.
When you get any lab results, if you'll post them here, we can help interpret them. Be sure to include reference ranges since those vary from lab to lab and have to come from your own report(s).
I would certainly attempt an elimination diet. Took me almost 4 months of doing so, but I'm on a careful, custom-made version of the Paleo diet that has stabilized my weight and my thyroid for over a year now. Given I live in a northern climate here in the US, I get my vitamin D in the warm months by sunlight exposure and supplement as outlined by my physician in the winter months.
As for your nodule, I concur with the others. Worrying over the nodule and biopsy results won't do you any good. If it meets the criteria to be examined, which it seems so, go do the FNA and let the news be the news. You can't control the outcome anyway. Focus on what you can, which is your diet, and try to get your thyroid in working order. On the off chance it is suspicious or outright cancerous (like mine was), the fortunate news is 95% of thyca is indolent, slow-growing, and can be treated, even if it has metastasis. I speak from experience when I tell you if it's suspicious or obvious thyca, they'll be calling you in a hot minute to set up surgery to take care of the problem.
Ok. I got some kid my labs back. They only checked my TSH. It was normal at 1.65. My vitamin D was low which we already knew. My ALT was elevated at 66 (high normal is 29.) 2 years ago it measured at 17. Other than that my CBC and metabolic panel was normal. I have my first endo appointment on the 27th. Hopefully they do a more thorough blood panel.
I am currently eliminating gluten as a start. It’s not easy at all!