mickeysgal: Sorry it took so long for someone to come back to this thread. Sometimes it's very active, and then other times we all get on with things and forget to check back!
To answer your questions:
1. If the thyroid is functioning normally, why would I need medication?
ANSWER: It's best if you start now collecting reports from your lab work. Thyroid issues are life long and if you change doctors or want to just have a reference for how you feel at a particular TSH/T4 level, then having copies of your thyroid lab work is good. I don't know who you get your bloodwork done with but most labs now have the ability for you to sign up through them and you can access all your results without having to bother your doctor's office. Anyway, you may be in the normal range; however, maybe you are closer to being subclinically hypo thyroid? A small dose would help with that. It could also be possible that treating you with synthetic thyroid hormone is a way to control the autoimmune aspect of it. I don't know any doctor that would give thyroid hormone to someone whose numbers look good because it can cause side effects if you become over medicated. 50mcgs could cause an issue if your numbers are good. Which is why I would try to see what your TSH is.
2. I take a lot of vitamin supplements...calcium (I'm over 50), multi vitamin, turmeric, fish oil, flax seed oil, vit d, biotin. Are any of these not a good mix with this medicine? I know I have to take the thyroid meds first thing in the morning, without any other supplements, but when I do take those supplements later in the day, are they ok to take in general, given I'm on thyroid medicine?
ANSWER: Remember that levothyroxine is not a 'medicine' per se, but a hormone. So it's not like an antibiotic, it's more like a birth control pill. The supplements you list are all OK with it but take any calcium 4 hours away. My endocrinologist did tell me that taking Vitamin D at the same time as the thyroid medication is okay. I have issues trying to remember everything to take. My top priority is the thyroid stuff so I get that in right away and generally forget everything else. I need to, myself, do better with that as I am over 50 too and Vitamin D deficient.
3. I'm a runner...I run half marathons and full 26.2 mile marathons. I have a 20 mile run scheduled for this weekend. I'm worried about starting this medicine and it making my heart rate increase/palpitations occur while I'm running. I talked to the doctor and they said it shouldn't cause an issue with this dosage. Does this sound right?
ANSWER: Well, I'm sure your run is now over. The thyroid meds could cause heart palps, but it might take a week at least before you notice that. Again, it all comes down to what your levels are pre-medication.
4. What kind of side effects can I expect once I start on this medicine?
ANSWER: If the dosage still keeps you in the normal range for thyroid levels, you might not notice a thing. If it pushes you closer to hyperthyroidism, you might notice being jittery, increased anxiety, more frequent stools, moodiness, etc. It's really individual. Some people can get VERY hyper and not have any change at all. Depends how sensitive you are.