CH. 16 PT 2. Thyroid & Parathyroid Gland

thyroid gland

butterfly-shaped gland in anterior neck on the trachea, just inferior to larynx

isthmus

median mass connecting two lateral lobes

follicles

hollow sphere of epithelial follicular cells that produce glycoprotein THYROGLOBULIN

colloid

fluid of follicle lumen containing thyroglobulin plus iodine and is precursor to thyroid hormone

parafollicular cells

produce hormone calcitonin

thyroid hormone (TH)

• body’s major metabollic hormone

• found in two forms (T3 and T4)

• affects every cell in body

• enters target cell and binds to intracellular receptors within nucleus

effects of thyroid hormone (TH)

• increases basal metabollic rate and heat production

• regulate tissue growth and development

• maintains blood pressure

T4

thyroxine

• major form that consists of two tyosine molecules with four bound iodine atoms

• inactive form of thyroid hormone

T3

triiodothyronine

• from that has two tyrosines with three bound iodine atoms

• active form of thyroid hormone

thyroid hormone storage and stimulus

thyroid gland stores hormone extracellularly in follicle lumen until triggered by TSH to release

7 steps involved in synthesis of thyroid hormone (TH)- see WS for reference

1. thyroglobulin is synthesized and discharged into follicle lumen

2. iodide is trapped: iodide ions (I-) are actively taken into cell and released into lumen

3. iodide oxidized; electrons are removed, converting it to iodine (I2)

4. iodine is attached to tyrosine: mediated by peroxidase enzymes (monoiodotyrosine/MIT) forms if only one iodine attaches, and (diiodotyrosine/DIT) forms if two iodines attach

5. iodinated tyrosines link together to form T3 and T4 (if one MIT and one DIT link, T3 is formed, if two DITs link T4 is formed)

6. colloid is endocytosed by follicular cells (vesicle is then combined with a lysosome)

7. lysosomal enzymes cleave T3 and T4 from thyroglobulin, then hormones are secreted into bloodstream (mostly T4 secreted, T3 is also secreted, but T4 must be converted to T3 at tissue level)

how is thyroid hormone transported and regulated

by thyroxine-binding globulins (TBGs), both T3 and T4 bind to target receptors

• TH release is regulated by negative feedback (falling TH levels stimulate release of thyroid-stimulating hormone (TSH)

• rising TH levels provide negative feedback inhibition on TSH

• TSH can be inhibited by GHIH, dopamine, and increased levels of cortisol and iodide

• hypothalamic thyropin-releasing hormone (TRH) can overcome negative feedback during pregnancy or exposure to cold, especially in infants

thyroid hormone effects on BMR

promotes normal oxygen use and BMR, calorigenesis, enhances effects of sympathetic nervous system

effects of thyroid hormone hyposecretion on BMR

• BMR below normal

• decreased body temp

• cold intolerance

• decreased appetite

• weight gain

• reduced sensitivity to catecholamines

effects of thyroid hypersecretion on BMR

• BMR above normal

• increased body temperature

• heat intolerance

• increased appetite

• weight loss

effects of thyroid hormone on carbohydrate/lipid/protein metabolism

• promotes glucose catabolism

• mobilizes fats

• essential for protein synthesis

• enhances liver’s synthesis of cholesterol

effects of thyroid hormone hyposecretion on carbohydrate/lipid/protein metabolism

• decreased glucose metabolism

• elevated cholesterol/tryglyceride levels in blood

• decreased protein synthesis

• edema

effects of thyroid hormone hypersecretion on carbohydrate/lipid/protein metabolism

• enhanced catabolism of glucose, proteins, and fats

• weight loss

• loss of muscle mass

effects of thyroid hormone on nervous system

• promotes normal development of nervous system in fetus and infant

• promotes normal adult nervous system function

effects for thyroid hormone hyposecretion on nervous system

• in infant, slowed brain development

• intellectual disability

• in adult, mental dulling, depression, memory impairment

• hypoactive reflexes

effects of thyroid hormone hypersecretion on nervous system

• irritability

• restlessness

• insomnia

• personality changes

• exophtalmos in graves disease

effects of thyroid hormone on cardiovascular system

promotes normal functioning of the heart

effects of thyroid hormone hyposecretion on cardiovascular system

• decreased efficeiency of heart’s pumping action

• low heart rate and BP

effects of thyroid hormone hypersecretion on cardiovascular system

• increased sensitivity to catecholamines can lead to rapid heart rate, palpitations, high blood pressure, and heart failure

effects of thyroid hormone on muscular system

promotes normal muscular development and function

effects of thyroid hormone hyposecretion in the muscular system

• sluggish muscle action

• muscle cramps

• myalgia

effects of thyroid hormone hypersecretion on muscular system

muscle atrophy and weakness

effects of thyroid hormone on skeletal system

promotes normal growth and maturation or the skeleton

effects of thyroid hormone hyposecretion on skeletal system

• in child, growth retaradation, skeletal stunting and retention of child’s body proportions

• in adult, joint pain

effects of thyroid hormone hypersecretion on skeletal system

• in child, excessive skeletal growth initially, followed by early epiphyseal closure and short stature

• in adult, demineralization of skeleton

effects of thyroid hormone in GI system

promotes normal GI motility and tone, increases secretion of digestive juices

effects of thyroid hormone hyposecretion on GI system

• depressed GI motility, tone, and secretory activity, constipation

effects of thyroid hormone hypersecretion on GI system

excessive GI motility, diarrhea

effects of thyroid hormone on reproductive system

promotes normal female reproductive ability and lactation

effects of thyroid hormone hyposecretion on reproductive system

• depressed ovarian function

• sterility

• depressed lactation

effects of thyroid hormone hypersecretion on reproductive system

• in females, depressed ovarian function

• in males, impotence

effects if thyroid hormone on integumentary system

promotes hormal hydration and secretory activity of skin

effects of thyroid hormone hyposecretion on integumentary system

• skin pale, thick, and dry

• facial edema

• hair coarse and thick

effects of thyroid hormone hypersecretion on integumentary system

• skin flushed, thin and moist

• hair fine and soft

• nails soft and thin

hyposecretion of TH in adults and lead to

myxedema

• symptoms: low metabolic rate, thick or dry skin, puffy eyes, feeling chilled, constipation, edema, mental sluggishness, lethargy

what happens when hyposecretion of TH is caused by lack of iodine

a goiter may develop.

• lack of iodine decreases TH levels, which triggers increased TSH secretion, triggering thyroid to synthesize more and more unusable thyroglobulin

• thyroid enlarges

hyposecretion of TH in infants leads to

cretinism.

• symptoms: intellectual diseases, short and disproportionately sized body, thick tongue and neck

most common type of hypersecretion of TH

Graves’ disease

what is graves disease

• autoimmune disease, body makes abnormal antibodies directed against thyroid follicular cells

• antibodies mimic TSH, stimulating TH release

• symptoms: elevated metabolic rate, sweating, rapid/irregular heartbeats, nervousness, and weight loss despite adequate food

• exophtalamos may result: eyes bludge as tissue behind eyes becomes edematous and fibrous

• treatment: surgical removal of thyroid or radioactive iodine to destory active thyroid cells

calcitonin

produced by parafollicular (C) cells in response to high Ca2+ levels

• antagonist to parathyroid hormone (PTH)

• no known physiological role in humans, but at elevated doses: can inhibit osteoclast activity and prevent release of Ca2+ from bone matrix, or stimulates Ca2+ uptake and incorportation into bone matrix

parathyroid gland

four to eight tiny yellow-brown glands embedded in posterior aspect of thyroid

• contain parathyroid cells that secrete parathyroid hormone (PTH)

• PTH is the most important hormone in Ca2+ homeostasis

• secreted in response to low blood levels of Ca2+

• inhibited by rising levels of Ca2+

target organs of parathyroid gland

• skeleton

• kidneys

• intestine

parathyroid gland functioning

• stimulate osteoclasts to digest bone matrix and release Ca2+ to blood

• enhances reabsorbtion of Ca2+ and secretion of phosphate (PO4³) by kidneys

• promotes activation of vitamin D by kidneys, which leads to increased absorption of Ca2+ by intestinal mucosa

what happens when there is a parathyroid gland tumor

hyperparathyroidism

what is hyperparathyroidism

caused by parathyroid tumor

• calcium leaches from bones, causing them to soften and deform

• elevated Ca2+ depresses nervous system and contributes to formation of kidney stones

• osteitis fibrosa cystica: easily fractured bones

what happens following parathyroid gland trauma or removal

hypoparathyroidusm

hypoparathyroidism results in

hypocalcemia (low calcium)

• results in tetany (sustained muscular contraction)

• respiratory paralysis

• death