There's a wide range of what's considered normal when testing for thyroid function. Your thyroid stimulating hormone (TSH) can be as low as 0.4 and as high as 4.5 and still be considered normal. That doesn't mean that you're normal, just that the lab test is normal. Therefore, it's important to look back historically at your previous TSH when you were feeling good and try to get your current TSH back to that range.
Of course, you don't want to take too much Synthroid (levothyroxine) such that you end up with side effects, eg tremors, shakes, anxiety, insomnia, weight loss, diarrhea, heat intolerance, palpitations => atrial fibrillation, and/or bone loss => osteoporotic fracture.
Undesired weight loss is another topic in and of itself. Besides too much thyroid medication (which can also cause fatigue, incredibly enough), we also worry about latent infections (TB & HIV come to mind) and undiagnosed cancer. Sorry, just telling it like it is. Of course, if you're not eating enough, you'll lose weight. So make sure that you're getting good nutrition, preferably in 3-6 meals a day. Make sure you get plenty of protein, vitamins & minerals plus fiber rather than just empty calories.
Not enough cortisol (Addison's disease) can also cause fatigue. But so can sleep apnea, heart failure, and a host of other issues. You're rather young but even hypogonadism or low testosterone might account for your symptoms.
Oh yes, let's not forget external stressors, such as work, family, friends, economy, etc. We all deal with that daily, some more, some less. But if you're having major issues, depression also comes to mind as accounting for weight loss, fatigue & insomnia. So make sure your doc takes a look at all of you. Good luck!