Hi Stephanie, yes, thyroid medication can lighten your period. I also had this with the levo. I'm older, so you know how it is, that docs are like, oh, it's bc you're getting older. But thyroid meds affects the pituitary's production of TSH. It isn't as if you can just separate out one pituitary function from another, if the pituitary is suppressed, it likely affects more than one area.
You're fortunate that you're young, as it's easier to recover. I've used acupuncture to help get things back on track throughout the last few years of severe thyroid disease. Please do read the many posts here on levothyroxin. I've seen a few posts where people have eventually been able to work with this medication, but tons of posts about horrible side effects, hot flashes, liver and kidney disease, arthritis, fibromyalgia, severe migraines, bone loss... I was horribly ill on this medication, and don't feel I'm particularly special in that I became so il. There are many clinical case studies on young women with no other health problems, showing that this medication causes liver lesions at the lowest dose after only one month.
Here's how it went for me... I was on the levo for over two years, it didn't work at any dose, I gained 50 pounds and I became so ill, I was bedridden. I weaned off with a glandular, but got fed up with manufacturing and formulation changes (across the board with these meds) and went off the meds altogether, in favor of a natural approach. I found amino acid therapy to be as effective as the meds (when I'm able to take it).
When I was on the meds, my period was lighter than usual. When I quit all the meds, my period stopped. Now, off all the meds for six months, it's come back and won't stop. I've got so many health issues, I can't even tolerate the amino acids at the dose I need, I know I'm hypothyroid, but I'm still feeling so much better without the meds... Two weeks on a period is just another thing, and at least it feel a little more normal now. It's just another thing, and it will resolve itself shortly.
I should say that prior to my thyroid attack my cycle was like clockwork. The hypothyroid can cause spotting and shorter time between cycles due to low progesterone, aside effect of low thyroid.
Because of the extensive influence the thyroid has on the entire hormonal system, I found a hormone cure book to be very helpful as a reference whenever my system gets wonky. The thyroid goes down, this eventually shuts down the adrenals, and other hormones... Down the line. So some help rebooting things is important.
Hope that helps give you a long term perspective so you can make good choices to help yourself heal. There are many posts about dietary and supplements for thyroid treatment.
Good luck honey.