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.

 

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