Describe the role of the hypothalamus in control of hormone secretion by the pituitary?

hypothalamus produces all the hormone secreted by the anterior and posterior pituitary gland and when changes occurs in the body that wants this hormones secreted ....hypothalamus releases GnRH ...which stimulates the anterior or posterior to release hormones to that effect...when the hormones are present in the blood stream...negative feedback mechanism is automatically activated to prevent over secretion.. GnIH ....gonadotropin inhibiting hormone is released to stop the production of the hormone in the hypothalamus which in turns affects its release at the anterior or posterior

hypothalamus sense body conditions and secret hormones that act on the anterior pituitary.  In other words anterior pituitary only secret hormones when hypothalamus give the order (in form of hormone) under normal condition.  Here are the hormones and what they control:

hypothalamus --------anterior pituitary
GnRH---------------------LH+FSH
CRH-----------------------ACTH
GHRH---------------------GH
TRH------------------------TSH+ PRL

these are just the stimulatory biggies, hypothalamus-pituitary control involves multi-layers stimulatory/inhibitory signals to determine the appropriate amount of hormone to secret.

Posterior pituitary (PP) is a bit different in a sense that it is under direct control by hypothalamus.  It is made up of neuro-endocrine cells that extend directly from the hypothalamus.  So, the hypothalamus does not require 'stimulatory' and 'inhibitory' hormones to control PP hormone release.  The two major hormone are Oxytocin and AVP