Endocrinology

ENDOCRINOLOGY

Endocrine Glands Ductless glands  Produce highly active chemical regulators-hormones  Hormones excreted into bloodstream and carried to target organs 

Functional Types of Hormones 

Releasing Factors  

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From hypothalmus Stimulate secretion of tropic hormones of anterior pituitary

Tropic Hormones: stimulate growth and activity of other endocrine glands Nontropic (effector) hormones: exert action on non-endocrine tissue

Control and Regulation of Hormones 

Primary control in hypothalmus Acted on by CNS or stress  Secretes “releasing factors” which stimulate anterior pituitary to secrete tropic hormones  Also secretes ADH and oxytocin (stored and released by posterior pituitary) 

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Anterior pituitary “master” gland- secretes tropic hormones  Also secretes Growth hormone, an effector hormones 

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Effector hormones exert feedback inhibition on hypothalamus or the anterior pituitary

THYROID HORMONES 

Thyroid Small gland wrapped around trachea  Secretes hormones that regulate metabolic rate and oxygen consumption  Also secretes calcitonin, a hormone that aid in calcium metabolism 

Thryotropin releasing hormone (TRH) made and released by hypothalmus  TRH stimulating synthesis and secretion of thyroid stimulating hormone (TSH) by anterior pituitary.  TSH stimulates synthesis and secretion of T3 and T4 by thyroid. 

Circulating thyroid hormones T3 (triiodothyronine) and T4 (thyroxine) – iodinated derivatives of tyrosine  Poorly soluble in plasma- transported in blood by thyroid-binding globulins (TBG) or albumin  >99% is bound to these proteins, <1% is “free”- free portion is active 

Hyperthyroidism 

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Symptoms: sweating, palpitations, insomnia, tremors, anxiety, exophthalmos Most common cause-Grave’s disease 

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Autoimmune disorder - antibodies to TSH receptors

Elevated T4 confirms diagnosis 

May need to measure “free T4” if abnormalities in thyroid binding globulins.

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If T4 levels are normal, may need to also measure T3 to rule out T3 thyrotoxicosis

Hypothyroidism Symptoms: dry skin, coarse/dry hair, swelling of eyes, constipation, lack of energy  Decreased T4 in most cases 

Elevated TSH to confirm diagnosis  Neonatal Hypothyroidism: cretinism 

Parathyroid Hormone (PTH)   

Also called parathryin Synthesized by parathyroid gland Key hormone in regulating calcium ion metabolism 

Acts on bone, kidney, GI tract to reabsorb or conserve calcium ions • Reduces urinary excretion of calcium • Increases bone resorption (release of calcium from bone • Increases synthesis of active form of Vitamin D- stimulates intestinal reabsorption of calcium

Hyperparathyroidism- causes hypercalcemia (other cause of hypercalcemia is malignancy)  Symptoms: 

Bone pain, osteoporosis  Kidney stones, flank pain, polyuria  Anorexia, constipation, vomiting  Anxiety, depression, fatigue 

Anterior Pituitary Hormones 

Growth Hormone (GH) or Somatropin Release stimulated by growth hormone releasing factor – suppressed by somatostatin  Promotes protein synthesis- stimulates bone growth  Decreased: dwarfism  Increased: Giantism  Diurnal variation: highest around midnight 

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Gonadotropins (FSH and LH) Induce growth of gonads  Induce secretion of gonadal hormones  Necessary for production of ova and development of sperm  Surge in LH is basis of home ovulation kits 

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Prolactin Initiates and maintains lactation  Useful in diagnosis, management, and follow-up of prolactinomas 

• Galactorrhea • Infertility

Posterior Pituitary Hormones 

ADH (vasopressin): Increases reabsorption of water by renal tubules  Decreased levels in diabetes insipidus 

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Oxytocin Contraction of smooth muscle  Used to induce labor  No medical reason to measure blood levels 

Adrenal Cortex Hormones 

Corticosteroids (Cortisol) Metabolism of proteins, carbohydrates, lipids  Diurnal variation, highest in morning  Diagnosis of adrenalcortical disorders, such as Cushing’s (increase) and Addison’s disease (decrease)  Often measure cortisol metabolites in urine (17-ketogenic steroids and 17hydroxysteroids) 

Testosterone  Aldosterone: increases sodium reabsorption in renal tubules 

Female Sex Hormones 

Ovarian Hormones – Estrogens Most potent is estradiol  Menstrual difficulties (with FSH and LH) to differentiate ovarian from pituitary causes  Estriol-no hormonal activity 

• Produced in 3rd trimester of pregnancy • Gives indication of fetal well-being • Sudden drop indicates fetal-placental distress

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Placental hormones 

Human Chorionic Gonadotroping (HCG) • Stimulates corpus luteum to produce progesterone and prevent menstruation • Produced by placenta shortly after implantation of fertilized egg • Pregnancy tests • Greatly influenced in hydatidiform moles • Increased in males with testicular cancer

Male Sex Hormones 

Testosterone Development of secondary sexual characteristics  Increased may indicated premature puberty in males or masculinity in females (virulism and hirsuitism)  Decreased in hypogonadism and some cases of infertility 

Adrenal Medulla Hormones 

Epinephrine and Norepinephrine Called catecholamines  Measured along with their metabolite vanillymandelic acid (VMA) when 

• Unexpected hypertension – rule out pheochromocytoma • Detect neuroblastoma in children