The Hypothalamus and Pituitary Gland

After childbirth, prolactin levels fall but the suckling response of an infant on the nipple produces an immediate increase in prolactin secretion which helps promote continued milk production.

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Another factor that can impact thyroid hormone secretion is caloric intake. If caloric intake is greatly elevated (particularly increased carbohydrate consumption), T3 levels go up and metabolism is increased. On the other hand, if caloric intake is drastically reduced as would happen during starvation or a strict diet, T3 levels decrease and metabolism goes down. These mechanisms are thought to be regulated at the level of the target tissues rather than in the thyroid gland, hence they are mediated by changes in the activity of the deiodinase enzyme.

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Another level of control of thyroid hormone activity occurs at the level of the target tissues. T3 is much more biologically active than thyroxine (T4). At the target tissue thyroxine can be deiodinated (one of the iodines is removed) to produce T3. By increasing or decreasing the activity of deiodinase thyroid hormone activity can be modulated.

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At the level of the hypothalamus, the thyroid hormones inhibit TRH production. In the anterior pituitary, they reduce the number of TRH receptors and inhibit TSH synthesis. the overall effect is to reduce TSH levels, which in turn lowers thryoid hormone synthesis and release.

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Deiodinase removes an enzyme from T4, producing T3.

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Growth hormone secretion exhibits a 24 hour (circadian cycle) with peak release occurring at night during sleep. stress increases growth hormone secretion.

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Growth in the human is not controlled by a single hormone, instead, growth requires the careful balance between growth hormone, thyroid hormones, insulin and the sex hormones, as well as proper nutrition. Nevertheless, growth hormone appears to be especially important in children, and its primary actions are essential for normal growth.

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If thyroid hormone levels are low, the inhibition is removed and more TSH is secreted to stimulate production and release of more thyroid hormones. Since the thyroid hormones have long half-lives, their concentrations in the blood remain relatively constant and do not tend to fluctuate.

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In underdeveloped countries, the most common cause of hypothyroidism is a lack of iodine in the diet. Without it, the hormone production is incomplete which results in a lack of negative feedback.

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Neurons in the hypothalamus secrete hormones that are taken up into the first capillary network. The blood leaving these capillaries converges on a portal vein that goes into the anterior pituitary and immediately enters the second capillary network.

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Oxytocin release increases when people do moral things

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Regulation of thyroid hormone secretion begins at the hypothalamus. The hypothalamus secretes the hormone thyrotropin-releasing hormone (TRH) into the hypothalamus hypophyseal portal system.

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TSH then stimulates the thyroid gland to secrete thyroid hormones, thyroxine (T4) and T3. Proper levels of thyroid hormones in the blood are regulated by a negative feedback system.

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The TSI's are not subject to the same negative feedback mechanisms as TSH and as a result constantly stimulate the gland.

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The anterior pituitary is composed of true glandular tissue capable of producing and secreting hormones.

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The body produces TSI's for unknown reasons. The TSI's circulate in the blood and bind to the TSH receptor on the follicular cells of the thyroid. The TSI's are agonists and therefore induce thyroid hormone release and enlargement of the gland (goiter).

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The hypothalamus has the essential role of functioning as the interface between the nervous system and the endocrine system. Since these two systems are key for regulation of body function it stands to reason that there needs to be communication between the two and the hypothalamus is key in this interaction.

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The hypothalamus receives input from higher brain centers and depending on the need, conveys that information to the endocrine system.

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The overall direct effect of growth hormone is to increase lipid utilization and the promotion of protein synthesis and growth.

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The posterior pituitary gland contains the axons and terminal ends of neurons that originate in the hypothalamus. The anterior pituitary gland contains cells that respond to signaling hormones from the hypothalamus. Neurons of the hypothalamus release a signaling hormone. These signaling hormones will stimulate anterior pituitary cells to either increase or decrease their release of other hormones that will travel through the body and act on specific target cells.

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ninety percent of the thyroid hormones are in the form of T4 which is the less active form.

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oxytocin plays a major role in both child birth and milk let down, both of which represent beneficial positive feedback mechanisms.

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the neurohormones are transported in vesicles down the axons to the posterior pituitary where they are stored in the axon terminals. When these same neurons receive the proper signal, they send action potentials down those same axons which stimulate release of neurohormones in the same way that action potentials in other neurons stimulate the release of neurotransmitters.

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the thyroid is composed of millions of small, spherical follicles that are composed of follicular cells and the lumens serve as reservoirs for the materials used to produce thyroid hormones.

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Know how the follicular cells synthesize thyroid hormones: page 355

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There are two outcomes from converting T4 to T3...

1. the bound hormones act as a reservoir for the thyroid hormones greatly increasing their half-lives (days). Indeed, thyroid hormone concentrations in the blood remain relatively constant and fluctuate little. 2. Second, it provides another level of control. By increasing deiodinase activity, the action of thyroid hormones can be increased without the necessity of increased production and release. Once in the cell, thyroid hormones interact with nuclear receptors that induce the expression of proteins directly involved in metabolism and oxygen utilization. In short, thyroid hormones increase the metabolic activity of tissue throughout the body.

what are the two hormones that are stored and released from the posterior pituitary gland?

ADH and Oxytocin.

True or False: Vasopressin is made by the posterior pituitary gland?

FALSE (posterior pituitary doesn't make any hormones.)

next to diabetes, thyroid disorders are the most common endocrine problems. Most thyroid disorders fall into one of two categories: ________ (increased thryoid activity) or __________ (decrease thyroid activity).

Hyperthyroidism Hypothyroidism

The pituitary gland has been referred to as the "master" gland due to its important role in regulating body functions. It is actually two glands in one, the anterior pituitary or ___________ and the posterior pituitary or _________.

adenohypophysis. neurohypophysis.

Prolactin is released from the ______ _______ and plays a major role in mammary gland enlargement during pregnancy in preparation for milk production.

anterior pituitary

What are some treatments for Hyperthyroidism?

beta blockers to decrease heart rate, propylthiouracil which decreases the production of thyroid hormones, and surgical removal of the thryoid followed by hormone replacement therapy.

If T4 and T3 levels are high, they feed back on the hypothalamus and the pituitary to _______ TSH release.

decrease

In the target tissues, T4 can be converted into T3 by the enzyme __________

deiodinase.

What inhibits prolactin?

dopamine

ADH is released when the osmolarity of the blood is ________ and stimulates the kidneys to reabsorb water.

elevated

Growth hormone is secreted by somatotrophic cells in the anterior pituitary gland under the control of _____ _____ _____ ______ and ____ _____ _____ _____ from the hypothalamus.

growth hormone releasing hormone. growth hormone inhibiting hormone.

There are unique populations of glandular cells that secrete the various anterior pituitary hormones in response to the hypothalamic hormones. (GNRH from the hypothalamus stimulates the secretion of FSH and LH from the anterior pituitary). This control is mediated through a special vascular arrangement between the hypothalamus and the anterior pituitary called the _______ ________ ______ _______.

hypothalamo hypophyseal portal system

Although the pituitary regulates many other endocrine glands, its activity is controlled by the _________ which sits directly above the pituitary gland.

hypothalamus

The anterior pituitary is derived from epithelial tissue and thus has glandular properties. The posterior pituitary is composed of neural tissues and is actually an extension of the _______ of the brain.

hypothalamus

the posterior pituitary is not composed of glandular tissue, it is an extension of the ________

hypothalamus

what are the direct actions of growth hormone?

increased uptake of amino acids and production of proteins by the cells, increased breakdown of fats and release of fatty acids by the fat cells, and increased synthesis and release of glucose into the blood by the liver, and decreased uptake of glucose by muscle and fat cells.

The pituitary is located in the sella turcica of the sphenoid bone and is connected to the hypothalamus by a stalk of tissue called the __________.

infundibulum

Why is this system beneficial?

it provides a direct connection between the hypothalamus and the anterior pituitary. Only small amounts of the hypothalamic hormones are required for control of the anterior pituitary.

rather than produce hormones, the posterior pituitary stores _____________ that are produced in the _________.

neurohormones hypothalamus

Parathyroid glands secrete ________ hormone which functions to _______ blood Ca++ concentrations.

parathyroid increase

Calcitonin will ________ blood Ca++ levels

reduce

What role does oxytocin have in childbirth?

stimulates the uterine smooth muscle to contract. As labor begins and the baby's head begins to push against the cervix of the uterus stretch receptors are stimulated that signal oxytocin release from the posterior pituitary. The oxytocin stimulates stronger uterine contractions which results in further stretch of the uterus and increased oxytocin release. This results in a positive feedback loop.

What is the connection between the hypothalamus and the posterior pituitary called?

the hypothalamic hypophyseal tract.

What secretes the hormone calcitonin?

the parafollicular cells of the thryoid

The _______ gland has the primary role of regulating our metabolism through the hormones it secretes.

thyroid

What is a portal system?

two capillary networks that are connected in series .

What is oxytocin's role in release of milk from the mammary glands?

Stimulation of the nipples by the nursing baby triggers oxytocin release which results in expulsion of milk from the mammary glands. The stimulus is removed when the baby stops nursing.

TRH stimulates cells in the anterior pituitary to secrete the hormone _____ ______ _______.

Thyroid Stimulating Hormone (TSH)

Graves' disease is a form of hyperthyroidism caused by antibodies called _______ ________ ________.

Thyroid Stimulating Immunoglobulins.