I have a history of hypothyroidism since I was 19 years old (35 years old now). I also had a several year bout with fairly severe CFS (was almost bed-bound). I've since recovered from the CFS after discovering the underlying hormone problem. Ever since that experience though and as I've gotten older, I've found that my body just doesn't operate like it used to. I've always been a long distance runner so this has all been extra upsetting. My thyroid levels are ideal/perfect with thyroid medicine, but if anything, I feel weaker on thyroid meds even though other aspects may be improved. I've always hated this feeling and wondered why I felt physically stronger (but less awake or motivated) OFF thyroid medication. I've ping-ponged myself off and on thyroid meds trying to decide which symptoms I'd rather deal with! (Don't do that -- the constant fluctuation is not good!) Well, I discovered initially through Dr. Izabella Wentz that thyroid medication including levothyroxine causes your body to use more carnitine in the body. This can lead to LESS carnitine stores/carnitine deficiency. I thought that was incredibly interesting, as I exhibited all of those symptoms and they were the ONLY lingering symptoms I still had from thyroid issues/previous CFS. She mentioned some studies in her article. I independently then also found so many studies done in the US and in Italy that tested lingering fatigue and weakness despite starting thyroid meds. They've done extensive studies on the dose of 1000mg twice a day and noticed a significant difference. Through some other studies, I've also seen the dose go all the way up to 2000mg twice a day. I started taking 1000mg twice daily and saw almost immediate improvements. My weakness disappeared, my running endurance increased, I stopped having this strange random muscle pain, and I even started sleeping better. I can't even tell you how much my sense of well-being has increased due to this supplement. I like the capsule form, so I take NOW brand 500mg capsules, just 4 caps per day. Lasts me about 15 days, but the price is reasonable enough it's not that big of a deal to me. I also want to address some reviews on here saying they heard L-Carnitine wasn't good for people with HYPOthyroidism - just hyperthyroidism. Again, you can look up Dr. Izabella Wentz's articles or the actual studies done on this topic, but L-Carnitine acts as a thyroid MODULATOR. That means, if you have hyperthyroidism, it will reduce your levels to more normal levels. If you have hypothyroidism, it has the potential to increase your thyroid hormones to more normal levels. The scientific community USED to think that it ONLY decreased thyroid hormones because that's what they happened to see with hyperthyroid patients. When in fact, it can do the opposite for hypothyroidism. It just modulates your levels to more normal levels, wherever you may be. My success may not be the answer for everyone! I need to mention I've spent YEARS working on my health incessantly and I feel like this was the last and final thing that made me feel like my normal self finally