MLAB 2401: Clinical ChemistryKeri Brophy-Martinez

Introduction to Endocrinology

Terms

• Endocrinology– Study of hormones and

disorders of these hormones

– Endocrine System– Hypothalamus & pituitary– Thyroid & parathyroid– Adrenals– Others (Islets of Langerhans,

ovaries, testicles, placenta)

Hormones

– Any substance normally produced by specialized cells in some part of the body, carried by the blood stream to another part, where it effects the body as a whole

– Vehicles for intracellular & extracellular communication

Characteristics of Hormones• Specificity

– Only target cells respond• Multiple actions• Variable half-life

– Often depends on solubility properties• Variable forms

– Depends on weight• Excretion rates

– Diurnal variation– Cyclic patterns– Stimulus response

Functions of Hormones

• Maintain homeostasis• Regulate growth and development• Promote sexual maturation, sexual rhythms and

facilitate reproduction• Regulate energy production• Adapt/adjust body to stressful/emergency

situations• Promote/inhibit production or release or other

hormones

Chemical Types of Hormones• Peptides/proteins

– Polypeptides or glycoproteins– Soluble in plasma– Interact with target cell membrane receptors to trigger a

second messenger to complete the specific action of the hormone.

– Short term effects• Amines

– Amino acid derivatives– Poorly soluble in plasma– Interact with membrane receptors of target cells– Provide long and short term effects

Chemical Types of Hormones

• Steroids– Composed of lipids (cholesterol)– Can transverse through the cell membrane– Produced by ovaries, testis, placenta, and adrenal

cortex– Insoluble in water– Long-lasting

Functional Types of Hormones

• Tropic– Originate from anterior pituitary gland– Specific for another endocrine gland

• Non-tropic or Direct effector– Secreted by non-pituitary endocrine glands– Act directly on peripheral tissue– Exert a feedback effect on the hypothalamus or

anterior pituitary gland

Hormone Receptors

• Located on cell membrane or within cell cytoplasm

• Binding of hormone to receptor initiates a signal

• Results in changes in gene expression• Ultimately causes a biological response

Regulation and Control of Hormones

• Occurs by controlling the rate of synthesis rather than the rate of degradation

• Primary control= Hypothalamus– Small gland next to pituitary gland– Connected to the pituitary by the “pituitary stalk”

• Pituitary Gland– Releases both tropic and effector hormones

Feedback Control• Negative feedback

An increase in the product causes a decreased in the system

Serves to stabilize a process Primary means of hormone

regulation• Positive feedback

An increase in the product causes an increase in the activity of the system

Sunheimer, R., & Graves, L. (2010). Clinical Laboratory Chemistry. Upper Saddle River: Pearson

Hypothalamus Hormones

Hormone Action

Thyrotropin releasing hormone (TRH) Releases TSH and prolactin

Gonadotropin releasing hormone (GnRH)

Releases LH and FSH

Corticotropin releasing hormone ( CRH) Releases ACTH

Growth hormone releasing hormone (GHRH)

Releases GH

Somatostatin Inhibits GH and TSH release

Dopamine Inhibits prolactin release

Anatomy of the Pituitary

• 3 distinct parts and their functions– Anterior pituitary (adenohypophysis)

• Hormones which target other endocrine glands• Forms the lactotrophs, somatotrophs, throtrophs,

corticotrophs and gonadotrophs

– Intermediate lobe (pars intermedialis)• Little functional capacity

– Posterior pituitary (neurohypophysis)• Stores and releases oxytocin and vasopressin (ADH)

Anatomy of Pituitary

Anterior Pituitary HormonesHormone Target Gland Classification Feedback

HormoneFunction

Luteinizing hormone (LH)

Gonad :Ovary/testes

Tropic Sex steroids OvulationTestosterone production

Follicle stimulating hormone (FSH)

Gonad :Ovary

Tropic Inhibin Ovarian recruitment

Thyroid stimulating hormone (TSH)

Thyroid Tropic Thyroid hormones (T4/T3)

Stimulates thyroid hormone production

Adrenocorticotropin hormone (ACTH)

Adrenal cortex Tropic Cortisol Stimulates synthesis & secretion of glucocorticoid hormones

Growth hormone (GH)

Multiple Direct effector Insulin-like growth factor

Stimulates tissue growth

Prolactin Breast Direct effector Unknown Secretion

LH & FSH

• Luteinizing hormone (LH):– Serves to promote ovulation, formation of corpus

luteum, and secretion of progesterone– Stimulates and secretes of androgens

• Follicle-stimulating hormone (FSH):– Stimulates growth of follicles, and along with LH,

secretion of estrogens and ovulation– Stimulates development of seminiferous tubules,

spermatogenesis

Actions of LH & FSH

Male Hormonal Control Female Hormonal Control

Sunheimer, R., & Graves, L. (2010). Clinical Laboratory Chemistry. Upper Saddle River: Pearson

TSH

• Tropic hormone• Used to confirm adult hypothyroidism

ACTH

• Target cell= adrenal cortex• Promote growth of adrenal cortical tissue• Stimulate the production of adrenal steroids

– Glucocorticoids– Mineralocorticoids– Androgen

• Diurnal variation– Highest levels between 6-8 am– Lowest levels between 6-11 pm

Growth Hormone

• Also called somatotropin• Peptide with direct effector functions• Release of GH is stimulated by GHRH

– Secretion occurs in pulse ~ every 2-3 hours– Peaks at the onset of sleep

• Inhibited by somatostatin

Growth Hormone• Amphibolic: influences both anabolic & catabolic processes• Allows effective transition from a fed state to a fasting state

without shortage of substrates• Directly antagonizes effect of insulin on glucose metabolism• Provides hepatic gluconeogenesis• Stimulates lipolysis• Enhances protein synthesis in skeletal muscle & other tissues• Stimulates production of insulin-like growth factors

Growth Hormone

Stimulators• Meals• Exercise• Sleep• Hypoglycemia

Inhibitors• Glucose loading• Epinephrine• Emotional/psychogenic

stress• Nutritional deficiencies• Insulin deficiency

Prolactin

• Functions in relation to reproduction– Breast growth during pregnancy– Milk secretory activity

• Direct effector hormone• Stimulated by thyrotropin-releasing hormone• Inhibited by dopamine

Posterior Pituitary Hormones• Posterior Pituitary is a storage region for 2

hormones• Oxytocin

– Function• Lactation

– Stimulates milk let down

• Stimulator of smooth muscle (uterine)– Synthetic oxytocin (Pitocin)

» Used to induce or enhance labor contractions

Posterior Pituitary Hormones

• ADH/vasopressin– Action

• Regulates water excretion in the renal tubules– Receptors for vasopressin found in the tubules

• Assists in water balance

– Hypothalamic osmoreceptors & vascular baroceptors regulate release of vasopressin from posterior pituitary.

References

• Bishop, M., Fody, E., & Schoeff, l. (2010). Clinical Chemistry: Techniques, principles, Correlations. Baltimore: Wolters Kluwer Lippincott Williams & Wilkins.

• Sunheimer, R., & Graves, L. (2010). Clinical Laboratory Chemistry. Upper Saddle River: Pearson .