I was diagnosed with a very underactive thyroid about 9 months ago.
Greyskythinker, i think that your symptoms tell you more than your blood test results, if you are having symptoms like fatigue, aching, feeling the cold, just feeling really miserable etc then I would seriously consider taking the medication, from what I've read some people with officially 'borderline' results really benefit. If you're not symptomatic don't take the pills (but maybe have regular blood tests and watch out for symptoms), also if your thyroid has been underactive for a while you might have been living with feeling under par for so long that you don't notice it - so take a good long look and maybe ask others that know you how they honestly think you are.
Thyroxine mimics your natural thyroid hormone, its main side affects are around dosage (ie if you take too much you get overactive thyroid symptoms). The idea is that you start on a low dose and build up (it can take a while to get it right). You can start taking it and then stop, although i think you need to cut down gradually, but in a lot of cases the underlying problem isn't one that goes away so it is likely that if you need the drug you will need it for life (this isn't always the case some times post partum hypothyroidism only needs short term treatment).
Being on a drug for life is scary, but if you need it it is worth it. The thing that I found most frustrating was the lack of any questioning as to why my thyroid had packed up - the only answer you seem to get is that it is just one of those things!
There are a lot of internet resources, but be warned that people tend to chat about the worst case scenario, for a lot of people (including myself) we take the pills and feel much better (eventually!).
Tinape, not sure about anxiety. Some people do have an overactive thyroid before it goes underactive but if yours is underactive now it sounds like the anxiety might be something else, I'm sure the stress of being diagnosed etc isn't helping either. Hope things settle down. Good luck