Thyroid Gland

Question 1

What hormones are produced in the thyroid gland and what do they do?

Answer 1

thyroid hormone- controls rate of metabolism and calcitonin- controls calcium metabolism

Question 2

What areas are affected by the thyroid gland?

Answer 2

all areas of the body except itself and the spleen, testes, and uterus

Question 3

What is the organization of the thyroid gland?

Answer 3

connective tissue separating cells into follicles

Question 4

What is Functional and structural unit of the thyroid?

Answer 4

thyroid follicle

Question 5

What are thyroid follicles?

Answer 5

cyst-like compartments formed by a simple cuboidal epithelium (follicular epithelium), contain a gel-like mass called colloid, oriented with their apical surfaces in contact with colloid; basal surface sits on a typical basal lamina

Question 6

What is the colloid used for?

Answer 6

storage site for T3 and T4 bound to thyroglobulin, extracellular storage of secretory products is unique to the thyroid gland

Question 7

What is thyrogluobulin?

Answer 7

iodinated glycoprotein; not a hormone but a scaffold or precursor for synthesis of thyroid hormones

Question 8

What are the structural features of follicular cells of the thyroid?

Answer 8

low cubodial to low columnar epithelium (they are tallest when stimulated) with typical junctional complexes at the apical region of cells, round nucleus sometimes with two nucleoli, slightly basophilic cytoplasm, Abundant vesicles (colloidal resorption droplets) are also present in the apical cytoplasm

Question 9

What are the structural features of the parafollicular cells?

Answer 9

individually or in small clusters at the periphery of the follicle, Pale staining, well developed Golgi and small, dense secretory granules containing calcitonin

Question 10

What stimulates secretion of calcitonin?

Answer 10

directly by high levels of calcium in the blood (not by hormones from pituitary)

Question 11

What does calcitonin do?

Answer 11

effectively functions to decrease calcium concentration in the blood

Question 12

What is a goiter?

Answer 12

Most goiters are enlargement of the thyroid gland due to iodine deficiency, Specifically thyrotropic cells of the anterior pituitary gland produce excess TSH. This leads to excessive growth of thyroid follicles & enlargement of the gland

Question 13

What s the general composition of thyroid hormones?

Answer 13

tyrosines linked together with iodine at 3 and 5 on the aromatic rings (T4 has 4 and T3 has 3); T4= 2 DIT, T3 = DIt and MIT

Question 14

What are the differences in T3 and T4 functionally?

Answer 14

major fraction secreted from the thyroid is T4; peripheral tissue (esp kidney and liver) derives T3 from T4 via deiodination, T3 is more active but has a much shorter half life (18h v 5-7days)

Question 15

Once secreted how does thyroid hormone travel in circulation?

Answer 15

bound to TBG, thyroxin binding globulin or thyroid binding globulin

Question 16

What are the 5 main systems or functions the thyroid hormone effects?

Answer 16

growth, heart, metabolism, BMR, and CNS

Question 17

How doe s thyroid hormone effect growth?

Answer 17

promote bone formation, promote ossification and fusion of bone plates, also maturation, essential for GH to be effective

Question 18

How doe s thyroid hormone effect the heart?

Answer 18

increase CO, HR, contractility, adrenergic receptors

Question 19

How doe s thyroid hormone effect metabolism?

Answer 19

increase GI glucose absorption, potentiate the effect of GH on gluconeogenesis and lipolysis, overall catabolic effect

Question 20

How doe s thyroid hormone effect BMR?

Answer 20

increase O2 consumption, body temp, induces synthesis of Na/K ATPase everywhere but brain, gonads, and spleen, overall increase in BMR

Question 21

How doe s thyroid hormone effect CNS?

Answer 21

inperinatal- essential for CNS maturation, hypothyroidism causes mental retardation; in adults- hypothyroidism causes listlessness, slow movement, impaired memory/mental capacity, hyperthyroidism- > hyperexcitability, hyperreflexia and irritability

Question 22

What is mainly stored in the colloid?

Answer 22

thyroglobulin

Question 23

What are the steps of synthesis of T3 and T4?

Answer 23

thyroglobulin synthesis, transport of iodine, organification of iodide, release of T3 and T4

Question 24

How is iodine transported?

Answer 24

actively to the lumen via Na/I cotransport down Na gradient where it is oxidized at the apical surface of thyrocytes

Question 25

What does thyroid peroxidase do?

Answer 25

oxidation of I1 to I2 on the apical surface of thyrocytes; 2 sequential reactions- iodination of tyrosines on TG and iodination of tyrosines by thyroid peroxidase yielding MIT and DIT; MIT and DIT remain attached to TG until stimulated by TSH

Question 26

What is the enzyme in peripheral tissue that converts T4 to T3?

Answer 26

deiodinase

Question 27

Binding of TSH on thyroid epithelial cells stimulates what?

Answer 27

synthesis of thyroglobulin, iodine transporter, thyroid peroxidase, and release of T3 and T4; also increases rate of endocytosis of colloid, bound T3 and T4 are liberated as free hormones into circulation via lysosomal enzymes after lysosome fuses with vesicle, relased from thyrocytes across basal lamina into fenestrated capillaries where they bind TBG

Question 28

When is TBG level increased?

Answer 28

during pregnancy, because estrogen levels double, to carry more hormone and meet demand of mom and baby

Question 29

How do thyroid hromones bind to their receptors?

Answer 29

as heterodimers with retinoid X receptor at sequence called TRE (thyroid hormone response element)

Question 30

What causes hyperthyroidism?

Answer 30

hyperplasia of entire gland due to autoimmune stimulation of TSH receptor

Question 31

What is graves disease?

Answer 31

excess thyroid hormone production caused by activation of TSH by antibodies produced in and outside of thyroid gland, activating mutations in TSH receptor or a goiter

Question 32

What are the clinical characteristics of hyperthyroidism?

Answer 32

elevated T4 in serum, increased uptake of radioactive iodide, suppressed TSH in serum, palpitations, tachycardia, increased appetite but weight loss

Question 33

What causes hypothyroidism?

Answer 33

loss of function thyroid tissue, biosynthesis defects, hypotrophic hypothyroidism, or peripheral resistance

Question 34

What are examples of loss of functional thyroid tissue hypothyroidism?

Answer 34

chronic autoimmune thyroiditis (hashimoto’s disease), antiboides against idodide transporter, deiodinase or thyroid peroxidase; idiopathic, postradioactive iodine treatment, or postthyroidectomy

Question 35

What are examples of biosynthesis defects?

Answer 35

inherited, iodine deficiency, antithyroid agents

Question 36

What are examples of hypotrophic hypothyroidism?

Answer 36

TSH or TRH deficiency

Question 37

What does calcitonin do?

Answer 37

fine tunes levels of calcium in the blood, suppressing bone resorption by inhibiting osteoclast activity preventing a release of calcium and phosphorus into the blood, it inhibits renal tubularreabsorption of calcium and phosphorus and increases excretion in urine;

Question 38

How is calcitonin used clinically?

Answer 38

treating hypercalcemia and pagets disease (bone remodeling disorder), and osteoporosis

Question 39

What are the symptoms of hyperparathyroidism?

Answer 39

corneal calcification, thirst due to dehydration, mental confusion, headache, coma, cardiac calcification, arrhythmias, ectopic calcification in liver and pancreas, renal calculi, polyuria, renal failure, bones painful and fragile with erosions

Question 40

How is hyperparathyroidism diagnosed? treated?

Answer 40

measuring blood PTH and blood Ca level; restrict dietary calcium, surgically remove parathyroid glands, monitor calcium

Question 41

What are causes of hyperparathyroidism?

Answer 41

parathyroid gland adenoma (80%), diffuse parathyroid hyperplasia, malignant chief cells are not inhibited by Ca and PTH secretion is not regulated

Question 42

What does hyperparathyroidism lead to?

Answer 42

osteomalacia in adults, rickets in children

Question 43

What is the result of hypoparatyroidism? What causes it?

Answer 43

hypocalcemia; complications of thyroid and parathyroid surgery, idiopathic hypoparathyroidism (autoimmune disease), pseudohypoparathyroidism which is congenital parathyroid resistance

Question 44

What are the symptoms of hypocalcemia?

Answer 44

cataract formation, mood changes, depression, convulsions, cardiac arryhtmias, spasms of skeletal muscle, muscle cramps, painful bones, numbness of hands and feet, paresthesia

Question 45

What does vitamin D deficiency cause?

Answer 45

rickets in children with bone deformities and bowed long bones, osteomalacia in adults; both result from impaired mineralization of bone matrix

Question 46

What can cause vitamin D deficiency?

Answer 46

combo of low sunlight exposure and decreased dietary intake, liver or kidney damage can cause it too

Question 47

What is type 1 Vitamin D resistance?

Answer 47

aka psuedovitamin D deficiency, caused by a defect in 1-a-25hydroxylation of 25-OHD3, treated by oral 1,25-OHD3

Question 48

What is type 2 Vitamin D resistance?

Answer 48

hereditary vitamin D resistant rickets, due to true end organ resistance, inherited autosomal recessive; several different mutations of receptor