It is most accurate to think of the hypothalamus, and therefore the brain, as the true “master gland” of the endocrine system.
The hypothalamus communicates directly with the pituitary gland, which then influences a wide variety of endocrine glands and body tissues through the release of a host of different hormones.
If the brain/hypothalamus is the legislative branch of the endocrine system, then the pituitary gland is the executive.
In this lecture we will examine all the other endocrine glands—some of which report directly to the pituitary gland, and some of which are more independent.
The Pineal Gland
Secretes the hormone melatonin.
The synthesis and release of melatonin is…
Stimulated by darkness
Inhibited by light
Levels of melatonin rise and fall on a circadian cycle (daily cycle).
The R group on the amino acid tyrosine is modified to make triiodothyronine (T3) and thyroxine (T4).
Some hormones are just modified amino acids!
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Thyroid Hormones: Function of T3 and T4
Important in proper development of organisms
Important in metabolism and maintaining homeostasis
An increase in T3 and T4 increases the rate of oxygen consumption and therefore cellular metabolism.
This helps maintain normal blood pressure, heart rate, etc.
Hypothyroidism in infants and during childhood is called cretinism, with the symptoms of stunted growth and intelligence.
Hypothyroidism in adults produces weight gain, lethargy, and a low tolerance for cold.
Hyperthyroidism produces high body temperature, sweating, weight loss, and high blood pressure.
Goiter is the enlargement of thyroid due inadequate amounts of iodine in the diet.
The reason we have iodized salt
Negative Feedback: How Levels of T3 and T4 Are Controlled
Homeostasis with regards to T3 and T4 is achieved through negative feedback.
The hypothalamus secretes TSH releasing hormone (TRH), causing the anterior pituitary gland to secrete TSH.
TSH does two things:
It tells the hypothalamus to stop producing TRH (negative feedback).
It tells the thyroid to produce T3 and T4.
T3 and T4 go to the hypothalamus and tell it to stop producing TRH (negative feedback).
So, when TSH, T3, and T4 levels are high they provide negative feedback to the hypothalamus to inhibit TRH production; when TSH, T3, and T4 levels are low, the hypothalamus releases TRH.
Excessive urine (hence the name “diabetes,” which is Greek for “copious urination”)
Sugar in urine (hence the name “mellitus,” which is Greek for “honey”)
Reasons for the disease:
Not enough insulin being produced.
Loss of response to insulin by the target tissues.
High levels of glucose in the blood cause glucose to be excreted by the kidney, which draws water with it, causing constant urination and constant thirst.
Irony: blood is full of glucose, but the cells cannot get any.
Two Types of Diabetes
Type 1: juvenile diabetes
Occurs in children (average age is less than 10 years old)
An autoimmune disease: the immune system attacks pancreas cells, killing the beta cells which normally produce insulin.
The patient must take daily injections of insulin for entire life.
Type 2: adult onset diabetes
Appears after age 40.
90 % of all diabetes cases
Reduced response to insulin receptors in the target tissue
Individuals produce insulin, but the cells in the body do not respond to it.
Obesity is a major cause of this disease
Therefore, it can be controlled by diet and exercise.