Endocrine Tissues and Hormones

Endocrine Tissue	Hormone	Site of Action	Action
Anterior Pituitary	Growth HormAdrenocorticotropin Hormone (ACTH)one (GH) Thyroid-Stimulating Hormone (TSH) Follicle-Stimulating Hormone (FSH) Luteinizing Hormone (LH) Prolactin	Most cells of the body Adrenal Glands (cortex) Thyroid Gland Ovary/Testes Ovary/Testes Breast tissue	Stimulates cell growth Stimulates adrenal cortex to release cortisol and aldosterone Causes thyroid to secrete thyroid hormones Promotes growth of follicles within the ovary (female), sperm formation (male) Stimulates estrogen/progesterone release (female), testosterone (male) Encourages breast development, milk production in pregnancy
Posterior Pituitary	Antidiuretic Hormone (ADH) Oxytocin	Kidneys, blood vessels Uterus/Breast tissue	Causes kidneys to retain water, increases blood pressure Induces contractions during labor, milk expulsion
Pineal	Melatonin	Brain, immune system	Governs sleep/wake cycle
Thyroid Gland	Thyroxine (T-4), Triiodothyronine (T-3) Calcitonin	Most cells of the body Bone cells	Increases metabolism, chemical reactions Increases calcium deposition
Parathyroid Glands	Parathormone	Gut, kidney, bone	Increases calcium in the blood
Adrenal Glands (medulla)	Adrenaline and Noradrenaline	Many cells and tissues	Initiates fight-or-flight response
Adrenal Glands (cortex)	Cortisol Aldosterone DHEA	Many cells and tissues Kidneys, sweat glands Immune system, brain	Initiates fight-or-flight response Promotes sodium/water retention, increases potassium loss Precursor to androgens, promotes immune and mood balance
Pancreas	Insulin Glucagon	Most cells of the body Liver, fat, muscle	Promotes entry of sugar into cells, fat deposition Increases glucose production and release into the blood
Ovaries	Estrogens Progesterone	Sex organs, uterus, bone, brain Sex organs, uterus, bone, brain	Involved in sexual development (female), menstruation, builds uterine tissue, bone metabolism Involved in sexual development (female), menstruation, uterine secretion, pregnancy
Testes	Testosterone	Sex organs, brain	Involved in sexual development (male), libido, muscle development
Hypothalamus Although not considered an endocrine gland, the hypothalamus exerts control over the pituitary via neuro-hormones.	Corticotropin-Releasing Hormone (CRH) Growth Hormone-Releasing Hormone Growth Hormone-Inhibiting Hormone Gonadotropin-Releasing Hormone (GnRH) Thyrotropin- Releasing Hormone (TRH) Prolactin Inhibiting Factor (Dopamine)	Anterior Pituitary Anterior Pituitary Anterior Pituitary Anterior Pituitary Anterior Pituitary Anterior Pituitary	Stimulates release of ACTH Stimulates release of GH Inhibits release of GH Stimulates release of LH and FSH Stimulates release of TSH Inhibits release of Prolactin

Hypothalamus

Intimately involved with the endocrine system is the hypothalamus, controlled by secretory neurons that control the release of hypophyseal hormones from the pituitary gland. These include follicle-stimulating hormone (FSH), leuteinizing hormone (LH), thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), growth hormone (GH), prolactin, oxytocin, and antidiuretic hormone (ADH). These hormones tell their respective organs to produce estrogen, progesterone, testosterone, thyroxin, and adrenal cortical hormones.

Receptors

Endocrine hormones typically act to control intracellular functions by first combining with hormone receptors on the surfaces of cells or inside the cells. Binding of the hormone with the receptor turns on a cascade of reactions within the cell to accomplish a particular goal. These hormone receptors are very large proteins, and each cell may have in the order of 2,000 to over 100,000 hormone receptors depending on the cell’s function. Receptors are understandably highly specific for a single hormone. This allows one hormone to act on a particular target tissue with minimal cross reactivity.
     Amazingly, the number of receptors in a target cell is in constant flux. For example, the binding of a hormone with its corresponding receptor causes the number of receptors to change. In “down-regulation” the number of receptors is reduced, causing a decreased responsivesness to the hormone. In “up-regulation,” the number of receptors is increased with a corresponding increase in hormone sensitivity. This phenomenon is responsible for many of the clinical effects seen with medical intervention, including some of the tolerance that develops to longer term pharmaceutical treatments (and thus the need to adjust dosages) and the drug withdrawal symptoms commonly experienced when a drug or hormone treatment is discontinued. It may also help us to explain how therapies that balance the endocrine system can lead to long-term changes and bring patients to a new plateau in their wellness.

Feedback Mechanisms

Control of hormone secretions is accomplished by an internal control system. In most instances, a feedback mechanism is employed. The majority of feedback loops work as a negative feedback. In negative feedback, the endocrine gland has a natural tendency to over-secrete its hormone. The hormone then stimulates the target tissue to perform a function. Thus, the important factor is not the rate of secretion of the hormone but the rate that the target function is performed.
     When the target organ’s activity rises to an appropriate level, feedback is released to the endocrine gland to slow further secretion of the hormone. For example, the pituitary gland (endocrine gland) secretes thyroid-stimulating hormone, which acts on the target organ, the thyroid gland, to stimulate production of the thyroid hormones T3 and T4. The pituitary then monitors the circulating levels of thyroid hormones. When these levels are deemed by the pituitary to have reached appropriate levels, the release of TSH by the pituitary is curtailed. If the target organ (in this case the thyroid gland) were not to be able to produce adequate amounts of thyroid hormones, a healthy pituitary would secrete more and more TSH until the thyroid gland responded.