| THE HYPOTHALAMO-PITUITARY REGULATORY SYSTEM
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| Hormones of the posterior pituitary gland are distinct from those of the anterior pituitary
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| The pituitary gland is an oval organ weighing about 0.6 g, encased in a bony cavity of the skull (sella turcica) below the brain. It communicates with the hypothalamus via the pituitary stalk, which contains a complex array of axons and portal blood vessels (Fig. 37.3). The pituitary gland is divided into two lobes. The anterior lobe (adenohypophysis), accounting for ∼80% of the gland, may be seen as a target organ for endocrine hormones released from hypothalamic nuclei and transported via the median eminence and the portal circulation. The hormones of the posterior pituitary (neurohypophysis) are actually synthesized and packaged in the supraoptic and paraventricular nuclei of the hypothalamus and transported in granules along axons to the pituitary.
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| HORMONE MEASUREMENT - A MATTER OF TIMING |
Since hormones are involved with homeostatic mechanisms, their secretion is not constant. In response to a specific stimulus, most hormones are released in bursts ranging from single spikes (e.g. growth hormone, GH) to sustained release (e.g. insulin). Hormone secretion often conforms to strict biological rhythms, which may be at intervals of 1-2 hours (LH), 24 hours (cortisol) or 28 days (progesterone ). For these reasons, it is always necessary to standardize the time and status of the patient before sampling, and it is often necessary to perform multiple sampling or dynamic testing. |
| Figure 37.3 The hypothalamo-anterior pituitary regulatory system. Hormones of the posterior pituitary are synthesized and packaged in the supraoptic and paraventricular nuclei of the hypothalamus (1), transported along axons and stored in the posterior pituitary prior to release into the circulation. The anterior pituitary releasing or release-inhibiting hormones are synthesized in various hypothalamic nuclei (2) and transported to the median eminence. From there they travel to the anterior pituitary via a portal venous system. |
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