Thyroid

Question 1

Thyroid hormone provides long term regulation of what?

Answer 1

Core body temperature

Question 2

What does the thyroid hormone act on?

Answer 2

Metabolism! (not thermoregulatory mechanisms)

Question 3

What does the hypothalamic -pituitary-thyroid axis do?

Answer 3

Regulates thyroid hormone release from the thyroid gland

Question 4

How does the HPT regulate thyroid hormone release?

Answer 4

1. PVN releases thyrotropin releasing hormone (TRH) into portal capillaries and the anterior pituitary
2. Anterior pituitary release thyroid stimulating hormone (TSH) into general circulation
3. Thyroid gland cells respond to TSH by releasing thyroid hormone into circulation

Question 5

Where is the thyroid gland located?

Answer 5

Between the thyroid cartilage down to the 6th tracheal cartilage ring

Question 6

What is a follicle?

Answer 6

• functional unit of thyroid
• having simple cuboidal cells incorporating iodine and synthesizing thyroglobulin
• having a colloid interior where T3 and T4 are synthesized and stored with thyroglobulin

Question 7

Explain T3 and T4 synthesis

Answer 7

• combining tyrosine, iodine, and thyroglobulin
• dietary iodide binds to tyrosines of thyroglobulin forming MIT and DIT
• MIT coupled with DIT forms T3 and T4

Question 8

What does TSH do?

Answer 8

released thyroid hormones from follicular cells by endocytosis thru pseudopodia

Question 9

How does TSH release thyroid hormones?

Answer 9

TSH activates cAMP which does:

• endocytosis of colloid
• thyroidal uptake of sm. molecules
• intracellular trans. of proteins
• most steps of thyroid hormone storage, synthesis, and release
• removing the “I” from MIT and DIT

Question 10

What are the general functions of T3 and T4?

Answer 10

• T3 affects every organ
• acting as a modulator of cell function
• essential in development

Question 11

What is the role of T3 and T4 in maturation?

Answer 11

skeletal and nervous system development

Question 12

What is cretinism?

Answer 12

early thyroid disease

• abnormal bone and CNS development
• stunted growth, malformation of facial bones, mental retardation

Question 13

What is the T3 mechanism of action?

Answer 13

• T3/4 cross membrane via ATP-ase transporter

- T4 converts to T3 via iodothyronine deiodinase

Question 14

What does T3 enhance?

Answer 14

• BMR
• oxidative metabolism (except in brain, testes and spleen)
• increased oxygen consumption and greater ATP utilization
• mitochondrial activity in skeletal muscle, heart, kidney, and liver

Question 15

How does T3 act on metabolism?

Answer 15

• enhances BMR via increased O2 intake
• increases overall activity in muscles, organs, fat and thermoregulation via stored food and O2
• facilitates hormonal actions over glucose, lipids, and protein metabolism
• increases CO by up regulation of beta receptors

Question 16

What are activated in a response to cold and strenuous exertion?

Answer 16

SNS and adrenal medulla

Question 17

What is permissive action?

Answer 17

T3 increases the response to catecholamines, and maintains a metabolic rate for generation and mobilization of substrates involved in adrenal response

Question 18

What do catecholamines enhance?

Answer 18

• T4–>T3 conversion (select tissues)

- increase glucose formation and enhance metabolic turnover

Question 19

What is another action of T3?

Answer 19

• uncoupling of ATP synthesis in catabolism, energy is released as heat
• increases ion channel permeability, enhancing Na/K pumping and ATP hydrolysis (releases heat)

Question 20

Explain the negative feedback control of T3,T4 secretion

Answer 20

T3 and T4 determined by T3 feedback inhibition of synthesis and release of TRH and TSH

Question 21

TRH and TSH expression is ______ related to T3 and T4

Answer 21

inversely

- T3 down regulates TRH and TSH synthesis

Question 22

Describe T3 feedback inhibition in the pituitary gland

Answer 22

T3 feedback inhibits TSH secretion by reducing anterior pituitary’s sensitivity to TRH (downgrades its synthesis and release)

Question 23

What does feedback on the HTP axis include?

Answer 23

• T3,T4 conversion
• energy state (food intake/metabolism)
• temperature

Question 24

How do you increase thermogenesis?

Answer 24

manipulate thyroid feedback sensitivity

Question 25

Cold increases the release of _____ and increases ______ production.

Answer 25

TRH.....T3 and T4

Question 26

How does cold exposure affect adrenal release?

Answer 26

Epinephrine release is stimulated and lowers responsiveness of PVN cells to T3

Question 27

What is hyperthyroidism?

Answer 27

increased synthesis of T3 and T4

Question 28

What are some causes of hyperthyroidism?

Answer 28

- reduced sensitivity of feedback - antibodies to thyroid gland release excess T3 and T4 - -> excess T3 and 4 inhibits TSH release

Question 29

What are symptoms of hyperthyroidism?

Answer 29

- goiter (hyperplasia of thyroid cells) - heat intolerance - weight loss - exophthalmos (protrusion of eyes due to lid lag) - nervousness, tremor - irritability, insomnia - tachycardia, arrhythmias

Question 30

What is hypothyroidism?

Answer 30

- decreased synthesis of T3 and T4 | - decreased conversion of T4 to T3

Question 31

What are some causes of hypothyroidism?

Answer 31

- autoimmune destruction (often with thyroiditis) - iodine deficiency - dietary goitrogens (turnips, cabbages)

Question 32

What are the results of decreased T3, T4 secretion?

Answer 32

- less hypothalamus and pituitary negative feedback - T3 and T4 levels are low while TSH levels are high - increased colloid due to increased TSH activity

Question 33

What are some symptoms of hypothyroidism?

Answer 33

- colloid goiter - cold intolerance - fatigue, sluggishness, slow speech, lack of initiative - delayed tendon reflexes - bradycardia - high cholesterol - hair loss, dry and corse skin, husky voice - enlarged liver, kidney, and tongue - myxedema (non-pitting edema)