Endocrine Physiology

Bob Bing-You, MD, MEd, MBA

Medical Director

Maine Center for Endocrinology

What is physiology?

• A. Study of function in living matter

• B. Specific characteristics and mechanisms of the human body that make it a living being

• C. “The human being is actually an automaton.”

• D. All of the above.

Learning Objectives

• Explain normal feedback mechanisms for normal endocrine function

• Describe abnormal pathophysiologic states

• Define treatment approaches for such states

Ground Rules

• Case-based, common examples

• Feel to ask questions anytime

• Take a few breaks for Q and A

• Call or e-mail [bingyb@mmc.org] with confused thoughts, concepts, etc.

Major Areas

• Thyroid gland

• Adrenal gland function, steroid Rx

• Diabetes: glucose monitoring, insulin Rx

• Calcium homeostasis, vitamin D

• Pheochromocytoma

• Diabetes insipidus

Design Exercise

• New heat source for your house in the basement

• Need monitor system 3 floors above

• Thermostats not invented yet!

• Task= design system to regulate temperature in your house

Purpose of Endocrine System

• Principally concerned with control of different metabolic functions of the body [e.g., transport of substances through cell membranes]

• Hormonal effects can occur in seconds while others require days or weeks

What is a hormone?

• A chemical substance secreted into body fluids by one cell or group of cells

• Exerts a physiological control effect on other cells in the body

• General vs. local hormones

• General hormones may effect all cells [e.g. GH] or some effect specific tissues

Major Hormones

• Anterior pituitary– Growth hormone– Adrenocorticotropin [ACTH]– Thyroid-stimulating hormone [TSH]– Follicle-stimulating hormone [FSH]– Luteinizing hormone [LH]– Prolactin [PRL]

More Hormones

• Posterior pituitary: antidiuretic hormone [ADH]

• Adrenal– Aldosterone– Glucocorticoids [cortisol]– Testosterone, estrogen– catecholamines

Even More Hormones!

• Thyroid hormones: thyroxine [T4], triiodothyronine [T3]

• Pancreatic: insulin, glucagon

• Parathyroid: parathyroid hormone [PTH]

Chemistry of Hormones

• Two types– Proteins or derivatives of proteins or amino

acids [e.g., anterior pituitary, thyroid]– Steroid hormones [e.g., adrenal cortex, gonads]

• Circulate in minute quantities [e.g., one-millionth of a milligram]

• Bioassay vs. radioimmunoassay

What describes hormones effecting other local cells?

• A. Paracrine

• B. Endocrine

• C. Autocrine

• D. None of the above.

Bioassay

• Animal cell tissue system

• Plasma or some extract added to see effect

• E.g., testosterone

Immunoassay

• Sandwich-type

• Very quick [e.g., intra-op PTH]

• Minimal blood or serum required

Mechanisms of Action

• Activation of cyclic AMP system of cells• Need hormone receptor [key and lock]• Cyclic AMP forms as an intracellular

hormonal mediator [aka as “second messenger”]

• ACTH, TSH, FSH, LH, ADH, PTH, glucagon

• Effects can be instantaneous

2nd mechanism of action

• Activation of genes in target cells, causing formation of intracellular proteins

• Steroid hormone enters cytoplasm and binds to receptor proteins

• Enters nucleus to activate genes to form messenger RNA

• m-RNA promotes new proteins to be made• Delayed effect of mins [aldosterone] to days

Transport of Hormones

• Biologic effect due to free hormones• Binding globulins – “the buses”

– Thyroid Binding Globulin [TBG]– Sex-hormone Binding Globulin [SHBG]– Cortisol Binding Globulin [CBG]– Produced in liver

• What is assay measuring? “Total” [I.e., bound] vs. “Free”

Sandwich-type assays refer to:

• A. Dialysis method

• B. Ham and cheese

• C. Two antibodies

• D. Live cellular responses

Control of Hormonal System

• Tendency of each gland is to oversecrete

• Need method to prevent oversecretion

• Need method to stimulate production if undersecreting

• KEY is Negative Feedback

• Some exceptions [e.g., prolactotrophs and dopaminergic inhibitory fibers]

Key Points

• Hormones effect metabolism all tissues

• Instantaneous vs. prolonged effects

• Free hormones biologically active

• Negative feedback is KEY to normal homeostasis