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in order for the body to increase its core temperature, it must stimulate metabolic processes in order to generate heat.
the hypothalamus receives stimuli from thermoreceptors all over the body that temperature is falling. It secretes pituitary hormone releasing factors which causes the anterior lobe of hte pituitary gland to begin secreting TSH (thyroid-stimulating hormone) which in turn, stimulates the release of thyroxine
in the Thyroid gland, a hormone which stimulates the increase in metabolic processes, generating heat.
if the body is too cold, the hypothalamus releases pituitary hormone inhibiting factors, which then causes the inhibition (decrease in secretion, NOT total inhibition) of all resulting hormones (i.e. TSH and thyroxine), causing all metabolic processes to slow down, generating less heat.
i hope this makes sense :)