explain the negative feedback system by which the secretion of thyroxin from the thyroid gland is controlled?

The thyroid gland secretes thyroxin (which controls the metabolic rate) into the bloodstream. Falling levels of thyroxin stimulate receptors in the brain to signal the hypothalamus to release a hormone (TRH - thyroid releasing hormone) that acts on the pituitary gland to release thyroid-stimulating hormone (TSH) into the blood. TSH acts on the thyroid, causing it to increase production of thyroxin.
Once the levels of thyroxin are restored these levels no longer stimulate the receptors in the brain/hypothalamus and so no more TSH is released.

Hypothalamus->TRH->Anterior lobe of Pituitary->TSH->Thyroid->Thyroxin.
When thyroxin levels are low the whole sequence above takes place.  When thyroxine levels are "normal" the thyroxine inhibits the release of TRH & TSH  = negative feedback.