It's not enough to say "T3 and T4 were within range"....... can you tell of if those are Free T3 and Free T4 or if they are Total T3 and Total T4? There is a difference; total free's are considered obsolete and of very little value. In addition, the ranges are very broad, so even though you fall within the normal range, that may not be normal for you. Could you please tell us if they are free or total, and post the results, along with the reference ranges, as these vary from lab to lab, so must come from your own report.
In addition, do you know if you have Hashimoto's Thyroiditis? Hashimoto's is an autoimmune disease in which the immune system produces thyroid antibodies that attack and eventually destroy the thyroid.
If your immune system is not functioning properly, it will be harder for you to recover from things such as this.
Please post the information regarding your T3 and T4 - maybe you are losing weight because you are over medicated; but we can't know that without knowing your actual levels.