Intro to Endocrinology Flashcards
Group 1 Hormones:
- Types
- Solubility
- Transport proteins?
- Plasma half-life
- Receptor
- Mediator
-
Types
- Steroids, iodothyronines
-
Solubility
- Lipophilic
-
Transport proteins?
- Yes
-
Plasma half-life
- Long (hours to days)
-
Receptor
- Intracellular
-
Mediator
- Receptor-hormone complex
Group 2 Hormones:
- Types
- Solubility
- Transport proteins?
- Plasma half-life
- Receptor
- Mediator
-
Types
- Polypeptides, proteins, glycoproteins, catecholamines
-
Solubility
- Hydrophilic
-
Transport proteins?
- Rarely
-
Plasma half-life
- Short (minutes)
-
Receptor
- Plasma membrane
-
Mediator
- cAMP, cGMP, Ca2+, metabolites of complex phosphinositols, kinase cascades
Examples of Group 1 hormones:
- Steroids –
- Thyroid hormone –
- Steroids – e.g. cortisol, aldosterone, testosterone (androgens), estrogens, progestins, calcitriol
- Thyroid hormone – thyroxine (T4) and triiodothyronine (T3)
Examples of Group 2 hormones:
- Peptides
- Proteins
- Glycoproteins
-
Peptides:
- oxytocin, vasopressin (ADH), angiotensins, somatostatin, thyrotropin-releasing hormone (TRH), gonadotropin-releasing hormone (GnRH), gastrin
-
Proteins:
- insulin, glucagon, ACTH, growth hormone, prolactin, CRH, GHRH, PTH, calcitonin, CCK, secretin
-
Glycoproteins:
- TSH, and the gonadotropins (FSH, LH, hCG)
Pearls
- Steroids all synthesized from some form of ________
- Thyroid hormones contain ______
- ________ generally do not have tertiary structure whereas ________ do.
- Glycoproteins all have same _____ subunit – specificity is in ____ subunit
- Steroids all synthesized from some form of cholesterol
- Thyroid hormones contain iodine
- Peptides generally do not have tertiary structure whereas proteins do
- Glycoproteins all have same alpha subunit – specificity is in beta subunit
Pearls: Describe the transport (bound or free) of the following hormones
- Thyroid hormones
- Steroid hormones
- Peptides
-
Thyroid hormones are bound >99.5% to TBG, transthyretin, and albumin
- plasma half-life is VERY long (T4 is 6 days),
- metabolic clearance very slow
-
Steroid hormones are bound 90-98% to plasma proteins (e.g. cortisol to CBG; testosterone to SHBG)
- half-life (~30-60 min) is shorter than thyroid hormones but still longer than most peptides and proteins
-
Most peptides and proteins circulate only in the free form
- half-lives are very short (e.g. <15 min)
- clearance rate is high
Pearls: Steroidogenesis
- What is the rate-limiting step for all steroidogenic pathways?
- Expression of different enzymes determines ….
- G-protein coupled receptor expression on the cell membrane determines ….
- Rate-limiting step for all steroidogenic pathways is steroidogenic acute regulatory (StAR) protein mediation of cholesterol uptake from the cytosol to the inner mitochondrial membrane where P450scc is located
- Expression of the different enzymes determines the major steroid product of each gland
- G-protein coupled receptor expression on the cell membrane determines which circulating secretagogue the gland will respond to
- e.g. ACTH, Angiotensin II, FSH, LH
What are the different types of cell signaling?
-
Endocrine
- seretion into a blood vessel
-
Paracrine
- secreted hormones act on adjacent cells
-
Autocrine
- secreted hormones act on orignal cell
- Neurotransmitter
- Neuroendocrine
Pearls: Hormone Mechanism of Action
- What does the chemical nature determine?
- What determines the specificty of hormone action?
- What mediates thyroid hormones?
- What mediates steroid hormones?
- What mediates peptides?
- Chemical nature of a hormone or a drug determines its mechanism of action
-
Expression of the appropriate receptor in tissue determines which hormones will act on that tissue
- specificity of hormone action
- Thyroid hormones act via nuclear receptors
- increase transcription and translation (slow)
- Steroid hormones act on cytoplasmic (e.g. cortisol) or nuclear (e.g estrogen) receptors
- increase transcription and translation
- Peptide hormones and catecholamines act on cell surface receptors and activate secondary messengers (rapid).
Give an example of how hormones are activated peripherally:
- T4 activated to T3 in target tissue
- Testosterone (T) activated to dihydrotestosterone (DHT) in target tissue
- Conversion of androgen to estrogen in tissue
- e.g. breast tumors and adipose tissue
Describe peripheral actvivation of thyroid hormone:
- Thyroxine (T4) and triiodothyronine (T3) readily diffuse through the cell membrane
- Much of the T4 is deiodinated to form T3:
- which interacts with the thyroid hormone receptor of the thyroid hormone response element of the gene.
- bound as a heterodimer with a retinoidX receptor
-
This causes either:
- increases or decreases in transcription of genes that lead to formation of proteins
- thus producing the thyroid hormone response of the cell
- Thyroid hormone acts on several different systems via mRNA

What is the precursor to estrogen?
androstenedione (A)
- What is the function of aromatase?
- What is the function of 17ß hydroxysteroid dehydrogenase?
- Aromatase mediates androstenedione (A) ⇒ estrone (E1)
- 17ß hydroxysteroid dehydrogenase mediates estrone (E1) ⇒ estrodial (E2)
How is estrogen important in breast cancer?
- Aromatase expression and enzyme activity in extraovarian tissues increases with advancing age
- Aromatase activity in skin and subcutaneous adipose fibroblasts ⇒ formation of systemically available estrone (E1)
- some estradiol (E2)
- Circulating A to E1 conversion in undifferentiated breast adipose fibroblasts
- Subsequent conversion of E1 to E2 in malignant epithelial cells provide high tissue concentrations of E2 for tumor growth
What is a potential therapy for breast cancer in postmenopausal women?
- aromatase inhibitors
- selective estrogen receptor modulators (SERMs)
What blocks the production of DHT?
- antiandrogens
- 5-α reductase inhibitors

Describe the simple negative feedback loop for hormone control:
- A sensor detects some regulated variable and responds by modulating its secretion of a hormone
- pancreatic islet cell senses glucose and releases insulin
- This hormone, in turn, acts on a target to modulate its production of another hormone or a metabolite, which may affect a second target
- In addition, the other hormone or metabolite feeds back on the original sensor cell

Describe negative feedback via hierarchical control:
- Under the influence of the cerebral cortex, the hypothalamus releases CRH, which stimulates the anterior pituitary to release ACTH, which, in turn, stimulates the adrenal cortex to release cortisol
- The cortisol acts on certain effector organs
- In addition, the cortisol feeds back on both the anterior pituitary and the hypothalamus

When would positive feedback occur?
- Late follicular and ovulatory phases of the menstrual cycle
- Oxytocin release during pregnancy and parturition

Describe the positive feedback in the late follicular and ovulatory phases of the menstrual cycle:
- high levels of estradiol
- cause greater secretion of the hypothalamic releasing hormone and trophic hormones in that system
- results in surge in pituitary hormone release
- responsible for ovulation at midcycle
Describe the physiologic effects and regulation oxytocin release:
-
release of oxytocin is stimulated by:
- distention of the cervix toward the term of pregnancy
- contraction of the uterus during parturition
- signals are transmitted to the paraventricular (PVN) and supraoptic (SON) nuclei of the hypothalamus
- provide a positive feedback regulation of oxytocin release
-
responsiveness of the uterine muscle is enhanced by:
- increased number in oxytocin receptors
- increase number in gap junctions between smooth muscle cells
- increased synthesis of prostaglandins
-
suckling of the nipple of the lactating breast also stimulates oxytocin release
- afferent sensory signals elicit an increase in oxytocin release into the circulation
What is primary hyperfunction?
- **too much production of the target **gland hormone that exerts the important systemic effect
- usually caused by a neoplasm (often a benign adenoma) of the gland of origin that has lost its normal control
- produces too much of its hormone in the absence of its normal stimulation (autonomous secretion)
- Because of negative feedback, the normal controller of the hormone is suppressed
What are examples of primary hypersecretion?
-
Adrenal Cushing’s syndrome:
- cortisol overproduction from the adrenal adenoma (usually) that suppresses ACTH release from the pituitary
- Therefore, diagnosis is made by measuring an increased cortisol and decreased ACTH
- also called “ACTH-independent Cushing’s syndrome”
-
Graves’ disease (primary hyperthyroidism):
- Excess thyroid hormone production independent of TSH
- Therefore, diagnosis is made by measured increased thyroid hormone and decreased TSH
What is secondary hyperfunction?
- too much production of the hormone that exerts the important systemic effect because it is being “told” to by an overproduction of the normal stimulator of that hormone
-
IMPORTANT:
- Although the normal stimulator of the hormone can be increased, it is often “within the reference range” for that hormone.
- However, it should be suppressed if the target gland hormone is increased, so the normal hormonal stimulator of the target gland is “inappropriately normal” or “inappropriately not suppressed”