Interesting. I've never seen this before to this extent; however, if you are not having any symptoms of hyperthyroidism (as I suspect that you are not since your free T4 level is normal) it is likely that you would not need to take any medications and your TSH can be rechecked periodically.
TSH is generally quite sensitive to the T4 and T3 needs of the body. If the body is sensing too much or too little the TSH can change dramatically in response.
I would recommend that, even though your T4 is normal) you have a full thyroid hormone profile done including T3 levels to see if your T3 level is elevated. If this were true it could cause your TSH to drop (although, again, if your T3 was elevated I would expect symptoms of hyperthyroidism).
It is probably a good idea for you to be followed by an endocrinologist since this is a little outside the realm of a "typical" thyroid patient.
Advice and opinions given are based on the information provided and must not be considered an official medical recommendation. The limitations of the internet prevent an appropriate evaluation. Always consult your doctor if there is concern.