Hashimoto's. Tsh: sep 0, 16* (0, 3-4) now 0, 16*. Ft4 sep 21, 4 (10-26) now 14, 4. T3 sep 2, 47 (1, 2-2, 8). Ft3 now 4, 9 (3, 3-6, 1).Sep 150mcg euthyrox, now 100mcg euthyrox & 10mcg liothyronine. Why is my TSH low?

Likely too much meds. It appears you've been over medicated and that your thyroid is at a high level of function for you. Even if the thyroid levels are in normal range, they are still very robust and just came down from much higher levels. It would take time for an adjustment to affect all levels and so these can be why TSH is low. If it persists, another visit to your doctor may be needed to lower meds. Good luck.
Opposite. Tsh originates from the pituitary gland. It stimulates the thyroid to produce t4. Unbound or free T4 is convered by your body to active form t3 (liothyronine). Too much T4 either produced from the thyroid or from thyroid med causes a negative feedback effect. It tells your pituitary gland not to produce tsh. So it goes down. This is why your dosage was probably decreased because your TSH was low.