The hypothalamus exerts control over the endocrine system by directly triggering activity in the

The hypothalamus is a part of the central nervous system that is directly linked to the pituitary gland and regulates its activity.
The pituitary gland is key in regulating the whole endocrine system because it secretes hormones that control the releasing of hormones in other glands such as the adrenal glands, the gonads etc. 
The hypothalamus excretes releasing hormones which travel to the pituitary and start the excretion of another set of hormones that travel to other glands starting the production of their own hormones.
This forms a delicate system which has a moving balance and responds to external stimuli through the connection the hypothalamus has with other parts of the brain which process information that comes from the external and internal receptors ( such as heat, hunger, emotional state etc.).

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