Why is posterior pituitary gland not a ‘true’ endocrine gland?

Neurohypophysis (pars nervosa) also known as posterior pituitary, stores and releases two hormones called oxytocin and vasopressin, which are actually synthesized by the hypothalamus and are transported axonally to neurohypophysis. 

Posterior pituitary gland does not secrete any hormones of its own and hence is not a ‘true’ endocrine gland.