Hashimoto's is not the only thing to cause hypothyroidism though it is one of the most common. Though with hashi's the thyroid will splutter to life from time to time. Other causes of hypothyroidism include as we know yoyo dieting and some medications such as lithium ( there are others but I can't recall of the top of my head but I can look it up if anyone's interested)

The endocrine system can be tricky sometimes.

I have Hashi's and I find I swing between hypo most of the time and hyper occasionally. It's not much to worry about if well monitored. Eventually your thyroid will peter out and you'll be hypo all the time. In some cases in Europe they remove the thyroid entirely. Whether to be worried or not should be based more on your symptoms rather than your lab value. Hashi's can mimick some psychiatric disorders such as bipolar.

I am not sure about the cancer link BUT it is an autoimmune disorder so the theory is is once you have one it makes you more succeptable to others such as lupus or RA. It can also in rare cases progress to Hashimoto's encephalitis but only rarely