Lecture 20, The Endocrine System (part 2)

Question 1

The Thyroid Gland

Answer 1

The Thyroid Gland
the thyroid gland is a butterfly-shaped organ located anterior to the trachea, and inferior to the larynx
- thyroid gland tissue is mostly composed of thyroid follicles, which contains a large cavity filled with a protein-rich fluid called colloid
- production of thyroid hormone occurs in the colloid, and is largely dependent on the presence of iodine
the thyroid produces and secretes two thyroid hormones:
- thyroxine (T4) - contains four iodine molecules
- triiodothyronine (T3) - contains 3 iodine molecules
T4 and T3 are both non-water-soluble
- in the blood, they are bound to specialized transport proteins called thyroxine-binding globulins (TBGs), albumin, or other plasma proteins
- less than 1% is found free/unbound in the plasma
T4 and T3 can be converted into one another by a group of enzyme called deiodinases
- T4 is present in a higher concentration, and is converted into T3 within target cells

Question 2

T3 and T4

Answer 2

T3 and T4 have key roles in regulating basal metabolic rate and in thermoregulation
- their release in regulated by a negative feedback loop
low basal metabolic rate, [T3 or T4], or core temperature is sensed by the hypothalamus:
- hypothalamus releases TRH
- TRH stimulates TSH release from the anterior pituitary gland
- TSH travels in the blood to the thyroid
- stimulates T3 and T4 release from the thyroid
- T4 is converted into T3
- T3 has effects on peripheral tissues to increase basal metabolic rate
normal or high basal metabolic rate, [T3 or T4], or core temperature is sensed by the hypothalamus:
- inhibits TRH release and TSH release

Question 3

T3 and T4 (2)

Answer 3

T3 and T4 have key roles in regulating basal metabolic rate and in thermoregulation
- are sometimes referred to as metabolic hormones due to their regulation of resting energy consumption
T3 has widespread effects on several organs and tissue throughout the body
- most cells of the body have T3 receptors in their nuclei
T3 receptors are not present on the plasma membrane, and are instead found within the nucleus
- once bound, T3 initiates changes in gene transcription and protein synthesis
- theses changes help promote carbohydrate or lipid metabolism, to maintain or elevate cellular [ATP]
- increases metabolism and ATP consumption = increases thermogenesis

Question 4

Hyperthyroidism

Answer 4

hyperthyroidism: a condition in which the thyroid gland in overactive, and produces too much T3/T4
* thyrotoxicosis: a condition where the circulating [T3] and [T4] is too high, regardless of the source
* thyroid storm: a severe case of thyrotoxicosis that results in organ dysfunction

Question 5

Hypothyroidism

Answer 5

hypothyroidism: a condition in which the thyroid gland is underactive, and does not produce enough T3/T4 to meet the body’s needs
* hashimoto’s disease: an autoimmune condition wherein immune cells attack thyroid tissue, causing damage and impair the ability of the thyroid to produce thyroid hormones
* secondary hypothyroidism: decreased thyroid activity due to a decrease in the amount of TSH released from the hypothalamus; can lead to thyroid atrophy

Question 6

Symptoms of Hyperthyroidism

Answer 6

• heat intolerance or elevated core temperature
• hyperactivity
• unexplained weight loss
• changes in appetite, GI function, or neurological function (mood, cognition, attention)

Question 7

Symptoms of Hypothyroidism

Answer 7

• cold sensitivity or decreased core temperature
• hypoactivity and/or fatigue
• unexplained weight gain
• changes in appetite, GI function, or neurological function (mood, cognition, attention)

Question 8

Parathyroid Glands

Answer 8

the parathyroid glands (n = 4) are embedded on the posterior surface of the thyroid gland, and produce parathyroid hormone (PTH)
* PTH is a peptide hormone primarily responsible for controlling plasma [Ca2+]
PTH production is controlled by plasma [Ca2+] in a negative feedback manner
* if plasma [Ca2+] is low, PTH secretion increases
* if plasma [Ca2+] is high, PTH secretion decreases
increase PTH acts on the bones, kidneys and GI tract:
* increase osteoclast activity
◦ increase bone resorption to release Ca2+ into the plasma
* increase Ca+ resorption by the kidneys (less Ca2+ excretion in urine)
* increased intestinal absorption of Ca2+
PTH maintains plasma [Ca2+] in its physiological range

Question 9

Gonads and Sex Hormones

Answer 9

gonads are reproductive organs that have dual functions in the secretion of sex hormones and gametogenesis
* testes: male gonads that produce testosterone and sperm
* ovaries: females gonads that produce estrogen, progesterone, and ova
* both males and females have testosterone, estrogen, and progesterone, just in varying concentrations
◦ sex hormone production is regulated by
gonadotropin-releasing hormone (GnRH), released
from the hypothalamus

Question 10

Testosterone

Answer 10

testosterone is the primary hormones released by the testes, and is produced in smaller concentrations by the ovaries and adrenal glands
* testosterone is an androgen, which is a class of steroid hormones that contribute to normal growth and reproductive development
* testosterone is important in the development of the testicular reproductive system, maturation of male gametes (sperm), and the development of secondary sex characteristics (deepened voice, body hair, increased muscle mass)
testosterone production is regulated by FSH and LH release by the anterior pituitary
* FSH stimulates sertoli cells to produce inhibin, which is a protein that inhibits FSH release
◦ negative feedback
* LH acts on leydig cels, which are involved in the production of testosterone
* testosterone inhibits LH release
◦ negative feedback
* the testosterone released by leydig cells stimulates sertoli cells to produce sperm (spermatogenesis)

Question 11

Estrogens

Answer 11

estrogens are a group of hormones produced by the ovaries, and is produced in smaller concentrations by the testes and adrenal glands
* estrogens are also androgens
* estrogens are important in the development of the ovarian reproductive system, maturation of female gametes (ova), regulation of the menstrual cycle, maintenance of pregnancy, and the development of secondary sex characteristics (breast tissue development, increased adipose tissue)
estrogen production is also regulated by FSH and LH release
* FSH stimulates granulosa cells to produce inhibin
◦ negative feedback
* FSH also stimulates the growth of oocytes (precursor cell to an ovum)
* LH acts on theca cells, which are involved in the production of androgens
* the androgens produced by theca cells are converted into estrogens by granulosa cells
* estrogen inhibits LH release by the anterior pituitary
◦ negative feedback

Question 12

The Pancreas

Answer 12

the pancreas is a long slender organ located inferior to the liver, posterior to the stomach, and has both exocrine and endocrine functions
* exocrine functions: secretion of digestive enzymes into the pancreatic duct, where it is secreted into the duodenum of the small intestine (associated with digestive system)
* endocrine functions: secretion of insulin, glucagon, somatostatin, and pancreatic polypeptide (within the endocrine system)
pancreatic islets are groups of cells in the pancreas that contain cells responsible for producing endocrine hormones
the pancreatic islets contain four types of cells: alpha cells, beta cells, delta cells, and gamma or pancreatic polypeptide (PP) cells

Question 13

alpha cells

Answer 13

produce glucagon and make up ~20% of the islet cells

Question 14

beta cells

Answer 14

produce insulin and make up ~75% of the islet cells

Question 15

delta cells

Answer 15

produce somatostatin and make up ~4% of the islet cells

Question 16

gamma or pancreatic polypeptide (PP) cells

Answer 16

produce PP and make up ~1% of the islet cells

Question 17

Glucagon

Answer 17

glucagon is a polypeptide hormone released from alpha cells, and mainly functions to prevent hypoglycemia:
* increases the breakdown of glycogen into glucose (glycogenolysis)
* stimulates gluconeogenesis (the formation of glucose from other sources, like lactate and pyruvate)
* increases the synthesis of ketones (ketogenesis)
◦ ketones are produced during the breakdown of
fat molecules, and can be used in oxidative
phosphorylation to produce ATP
◦ main fuel source used by the brain in situations of
starvation, extenuating exercise, or extremely low
plasma [glucose] -> (diabetes)
glucagon secretion is primarily regulated in a negative feedback manner, in response to changes in plasma [glucose]
* decreased plasma [glucose] stimulates glucagon secretion
* increased plasma [glucose] inhibits glucagon secretion
glucagon is also responsive to hormonal and neural inputs
* example: increased glucagon release during sympathetic activation

Question 18

Insulin

Answer 18

insulin is a polypeptide hormone released from beta cells, and mainly functions to prevent hyperglycemia :
* increases the uptake and utilization of glucose by myocytes, adipocytes, and liver cells
* stimulates glycogenolysis (the storage of glucose in the form of glycogen)
* decreases gluconeogenesis and ketogenesis
insulin secretion is primarily regulated in a negative feedback manner, in response to changes in plasma [glucose]
* increased plasma [glucose] stimulates insulin secretion
* decreased plasma [glucose] inhibits insulin secretion
insulin is also responsive to hormonal and neural inputs
* example: increased insulin release during parasympathetic activation
-> insulin - increase with parasympathetic and inhibited with sympathetic (glucagon is the opposite)

Question 19

Somatostatin and Pancreatic Polypeptide Hormone

Answer 19

somatostatin produced by the hypothalamus inhibits the secretion of growth hormone
* somatostatin produced by the pancreas inhibits the secretion of insulin and glucagon
* In general, somatostatin regulates bodily functions by regulating the release of hormones
pancreatic polypeptide hormone also modulates the release of insulin and glucagon, and contributes to satiation after eating
* in the brain, causes the reduction of appetite
* in the GI tract, slows/inhibits gastric emptying to produce a longer lasting feeling of fullness or satiety
* is often released in conjunction with insulin during/after eating

Question 20

Diabetes Mellitus (type I)

Answer 20

diabetes mellitus is a condition involving dysfunction in insulin secretion or sensitivity
* type I diabetes mellitus is an autoimmune condition where immune cells attack the beta cells of the pancreas, resulting in inadequate insulin production and secretion
◦ patients with type I diabetes require insulin
injections to maintain glucose homeostasis
◦ type I diabetes is linked to genetics

Question 21

Diabetes Mellitus (type II)

Answer 21

type II diabetes mellitus occurs when cells in the body become resistant to insulin, and are no longer responsive to insulin secretion
* type II diabetes compromises ~85% of all diabetes mellitus cases
* type II diabetes is largely influenced by lifestyle factors, such as improper diet and inactivity
◦ about 80-90% of individuals with type II diabetes
are overweight or obese
* in many cases, type II diabetes may be reversed by moderate weight loss, regular physical activity, and a nutritious diet

untreated diabetes can lead to severe health complications such as diabetic ketoacidosis, kidney dysfunction, nerve damage, heart disease and/or stroke, vision loss, and more