Endocrinology

Overview of the endocrine system

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Endocrinology

The endocrine system is a collection of glands which produce hormones that are secreted directly into the bloodstream.

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Gland	Hormones produced	Effect exerted
Hypothalamus	see notes on hypothalamic-pituitary-end organ axis	see notes on hypothalamic-pituitary-end organ axis
Pituitary	see notes on hypothalamic-pituitary-end organ axis	see notes on hypothalamic-pituitary-end organ axis
Pineal	Melatonin	Regulates the sleep-wake cycle
Thyroid	Triiodothyronine (T3) Thyroxine (T4) Calcitonin	Thyroid hormones: Basal metabolic rate - ↑ Protein formation - ↑ Sensitivity to catecholamines - ↑ Stimulate bone growth in children Calcitonin: Opposes the effects of PTH
Parathyroid	Parathyroid hormone (PTH)	Activation of osteoclasts → calcium and phosphate release from bones Promotes calcium reabsorption and phosphate excretion in the distal convoluted tubule of the kidney Stimulates the synthesis of 1-alpha-hydroxylase, thereby increasing renal production of vitamin D → increased absorption of calcium and phosphate from the intestine
Thymus	Thymosin	Stimulates the development of precursor T-cells into mature T-cells
Adrenal	Adrenal cortex: Cortisol Aldosterone Androgens - mainly dehydroepiandrosterone (DHEA) Adrenal medulla: Catecholamines; epinephrine and norepinephrine	Cortisol: Blood glucose - cortisol stimulates hepatic gluconeogenesis, thereby increasing glucose levels Immunomodulatory - cortisol inhibits the production of IL-2, interferon-gamma and TNFa which suppresses inflammation but leads to immunosuppression as IL-2 is required to stimulate T-cell proliferation. Stress response - increases sensitivity to catecholamines Catecholamines: Stimulate the sympathetic nervous system, leading to a number of effects including raised heart rate and blood pressure. Aldosterone: Stimulates sodium reabsorption and potassium excretion by the kidneys DHEA: Converted into testosterone and oestrogen
Kidney	Renin Erythropoietin (EPO) Thrombopoietin (TPO)	Renin - converts angiotensinogen to angiotensin I EPO - stimulates the development of erythrocyte precursors into erythrocytes. TPO - stimulates the production of megakaryocytes; the precursor to platelets.
Pancreas	Insulin Glucagon	Insulin - binds to insulin receptors located on liver, muscle and fat cells and stimulates the uptake of glucose and potassium ions. Blood glucose levels - ↓ Glucagon - acts on the liver to stimulate hepatic gluconeogenesis (formation of glucose) and glycogenolysis (breakdown of glycogen). Blood glucose levels - ↑
Testes	Testosterone	Men and women: Development of secondary sexual characteristics (i.e. pubic and armpit hair). Anabolic effects - bone formation and muscle growth Men only: Development of penis and testicles Stimulates spermatogenesis Deepening of voice
Ovary	Oestrogen Progesterone	Women: Uterus - endometrial proliferation Cervix - increased production of cervical mucus → facilitates passage of sperm Vagina - proliferation of epithelium Breasts - stimulates breast growth Pubis - stimulates hair growth

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Clinical relevance:

Disorders of endocrine glands commonly involve hyper (over) or hypo (under) secretion of their respective hormone(s).

This can be due to one of three reasons:

Primary disease - a problem with the gland itself
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Secondary disease - a problem with the pituitary gland
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‍Tertiary disease - a problem with the hypothalamus

The foundation of diagnosing any problem with the endocrine system involves three steps:

Measuring hormone levels in the blood.
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If levels are low: stimulation test, if levels are high: suppression test.
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Localisation of the problem - this can be done via CT/MRI imaging or further blood sampling

*NOTE* - the principles above are really simplified and sometimes multiple tests may be required to diagnose the problem. We'll cover this in the relevant topics.