Introduction to Endocrinology

Endocrinology Introduction

• Endocrinology is a study of hormones and their effects, working with the nervous system to maintain homeostasis.
• This involves growth, development, reproduction, blood pressure regulation.
• It also controls ion and substance concentrations, and influences behavior.
• Endocrine glands secrete hormones, impacting target tissues

The Endocrine System

• The endocrine system, alongside the nervous system, governs homeostasis.
• Responsibilities include growth, development, reproduction, blood pressure control, ion concentration, and influencing behavior.
• Endocrine physiology entails hormone secretion and their downstream actions.
• Key endocrine glands include the hypothalamus, anterior and posterior pituitary, thyroid, parathyroid, adrenal cortex, adrenal medulla, gonads, placenta, and pancreas.
• The kidney also acts as an endocrine gland with endocrine cells dispersed throughout the gastrointestinal tract.

Endocrine Glands and Hormones

• The hypothalamus secretes hormones that control the pituitary gland activities.
• The anterior pituitary secretes many hormones like TSH, LH, FSH, ACTH, MSH, growth hormone, and prolactin.
• The posterior pituitary releases hormones like oxytocin and ADH (antidiuretic hormone).
• The thyroid gland produces T3 and T4 hormones, while calcitonin is produced as well.
• The parathyroid regulates calcium levels with the hormone PTH.
• The adrenal cortex makes cortisol, aldosterone, and adrenal androgens (like DHEA and androstenedione).
• The adrenal medulla generates norepinephrine and epinephrine.
• The testes produce testosterone.
• The ovaries produce estradiol and progesterone.
• The placenta creates hormones like HCG, hPL, estriol, and progesterone, for pregnancies.
• The pancreas secretes insulin and glucagon to regulate blood sugar levels.
• The kidney plays roles in producing 1,25-dihydroxycholecalciferol, and renin

Hormone Synthesis

• Hormones are classified into proteins/polypeptides, steroids, and amino acid derivatives.
• Protein/polypeptide hormones are initially produced as preprohormones, then proceed through prohormone stages.
• They are synthesized in the rough endoplasmic reticulum, and packaged in secretory vesicles.
• Release occurs through exocytosis, triggered by various stimuli (e.g., cAMP, calcium).

Hormone Mechanisms

• Hormone actions begin with binding to a membrane receptor, forming a hormone-receptor complex.
• This initiates various pathways, including G protein, adenylyl cyclase, phospholipase C, or directly coupling to catalytic receptors.
• Second messengers (cAMP, IP3) amplify and orchestrate physiologic effects.
• Hormones can trigger changes in membrane potential, Ca2+ entry, or enzyme activation, leading to multiple actions.

Hormone Regulation

• Negative feedback mechanisms in hormone systems prevent overactivity.
• The hypothalamus plays a role in regulating the pituitary hormones.
• Positive feedback loops, though less common, can cause hormone surges, like the pre-ovulation LH surge spurred by estrogen.

Receptor Regulation and Mechanisms

• Hormones manage receptor sensitivity by either upregulating or downregulating receptor numbers or affinity.
• Downregulation decreases receptors to avoid excessive hormone effects.
• Upregulation increases receptors to enhance effects.

Steroid and Thyroid Hormones

• Steroid hormones (e.g., cortisol, estrogen, testosterone) and thyroid hormones act by entering the cell.
• They bind to intracellular receptors, forming hormone-receptor complexes.
• The complexes, bound to DNA, create transcription factors, regulating gene expression and protein synthesis.

Other

• Specific hormones have individual onset and duration of effects ranging from seconds to months
• These mechanisms regulate cell functions like metabolism, growth, and reproduction.