Answer:
Immediate effect of high TSH is release of thyroxin.
Low TSH leads to release of low thyroxin.
Hypothalamus releases TRH (thyrotropin releasing hormone) and TRH acts on pituitary to release TSH.
Pituitary gland releases TSH.
TSH stimulates thyroid gland to release thyroxin.
Thus if there is low TSH in the blood, immediately, the pituitary enters into action releasing high TSH, TSH enters into action by triggering thyroid to release more thyroxin and thus maintains homeostasis of thyroxin within blood stream.
If there is high TSH in blood stream, immediately, the thyroid released more thryroxin, thus more thyroxin could be found in blood stream, high levels of TH (thyroxin) in blood signals pituitary to release less TRH and thus it triggers less TSH from hypothalamus, and thus less thyroxin again from thyroid gland. Thus high TSH in blood through low TSH and TH triggers to maintain homeostasis in blood.
Thus TH (thyroxin) has negative feedback on hypothalamus and pituitary glands.
Explanation: