Things that lower TSH:  (the terms “high”, and “decreased” are relative to individual circumstances and must be interpreted accordingly)

Too high T4 or T3 (free and/or total).

Decreased iodine intake.

Dysfunctional hypothalmic signalling coming from:

brain (ans, emotion, sleep-wake),

other hormones,

biome,

skull infections.

Simliar pituitary dysfunctions

INTERPRETING TSH LOW (depending upon lab, lower than approximately 0.5 mIU/L)

There are numerous cases when the T3 free and total are low, T4 free and total are low, and even Reverse T3 low, while the TSH is also low.  The most feasible explanation is that the pituitary gland is somehow not properly sensing or responding to these congruent stimuli.