Thyroid Hormones

Question 1

Which hormones are secreted by the Thyroid Gland?

Answer 1

T₄
T₃
Calcitonin

Question 2

What is T₄?

Answer 2

Prohormone

Question 3

Why is T₄ converted to T₃?

Answer 3

To exert their action
T₃ is more biologically active than T₄

Question 4

Compare T₄ and T₃

Answer 4

T₃ is more biological active
T₃ has greater affinity for thyroid hormone receptors
T₃ has a shorter half-life

Question 5

What is calcitonin?

Answer 5

Hormone responsible for calcium homeostasis

Question 6

Where is calcitonin produced?

Answer 6

Parafollicular C cells

Question 7

Where is reverse T₃ (rT₃) produced?

Answer 7

Made outside thyroid gland by removal of iodine atom from T₄

Constitutes 1% of circulating throid hormone

Question 8

What are the steps in thyroid hormone synthesis?

Answer 8

Step 1: Iodide uptake
Step 2: Oxidation
Step 3: Organification
Step 4: Generation of T₃ and T₄
Step 5: Release of T₃ and T₄ from thyroglobulin into circulation

Question 9

Describe the stages of Step 2 (oxidation) of thyroid hormone synthesis

Answer 9

Thyroid Peroxidase (TPO) oxidises iodide to active form iodine

Question 10

Describe the stages of Step 3 (organification) of thyroid hormone synthesis

Answer 10

Question 11

Describe the stages of Step 4 (Generation of T₃ and T₄) of thyroid hormone synthesis

Answer 11

TPO combines MIT and DIT to form T₃ or combine two DITs to form T₄

Question 12

Describe the stages of Step 5 (Release of T₃ and T₄ from thyroglobulin into circulation) of thyroid hormone synthesis

Answer 12

Tg is endocytosed from the lumen into the follicular cell
Tg is recycled for additional iodination

Question 13

How does deiodination occur?

Answer 13

T₄ is deiodinized by 5^’-deiodinase in peripheral tissues

Question 14

What are the products of deiodination?

Answer 14

T₃
rT₃
Inactive degradation products

Question 15

What is rT₃?

Answer 15

Modulates action of active T₃ by competing for receptor binding

Question 16

What is the role of rT₃?

Answer 16

Prevents the body from reacting strongly to T₃

Question 17

What is the Wolff-Chaikoff effect?

Answer 17

Excess iodide can inhibit thyroid peroxidase and reduce the thyroid’s ability to produceT₃ and T₄

Iodide is the form in which iodine is transported

Question 18

How is the Wolff-Chaikoff effect limited?

Answer 18

Excess iodide is excreted from the urine

Question 19

How are thyroid hormones transported?

Answer 19

99% of T₃ and T₄ bind to thyroid-binding globulin (TBG) proteins
Remainder is free in blood stream and biologically active

Question 20

What may cause an increase in TBG levels?

Answer 20

Pregnancy
Oral contraceptive use

Estrogen stimulate TBG production in liver

Question 21

What are the effects of TBG levels increasing?

Answer 21

Bound and total thyroid hormones rise
Free T₃ and T₄ are constant

Question 22

What may cause a decrease in TBG levels?

Answer 22

Hepatic failure (in patients using corticosteroid drugs)

Question 23

What are the effects of TBG levels decreasing?

Answer 23

Bound and total thyroid hormones fall
Free T₃ and T₄ are constant

Question 24

How do T₃ and T₄ cross the cell membrane?

Answer 24

Transmembrane thyroid hormone transporters

T₃ and T₃ are lipophilic yet cannot cross the plasma membrane

Question 25

Where do thyroid hormones exert their effect?

Answer 25

Bind to nuclear receptors to modulate gene expression

Question 26

How do thyroid hormones exert metabolic effects?

Answer 26

Upregulate Na⁺/K⁺-ATPase Promotes cellular uptake of glucose Increases formation and catabolism of proteins

Question 27

What effect does upregulating Na⁺/K⁺-ATPase have?

Answer 27

Increases cellular oxygen consumption Increase heat production Increases basal metabolic rate

Question 28

What effects do thyroid hormones have on the heart?

Answer 28

Rapid pulse Increased Cardiac Output

Question 29

Why can thyroid affect the heart?

Answer 29

Upregulate β1-adrenergic receptors in the heart which increases response to sympathetic stimulation

Question 30

What role does thyroid have in fetal development?

Answer 30

Enhances brain and bone development

Question 31

How does the thyroid hormone enhance bone development?

Answer 31

Enhances formation and growth of osteoblasts Acts on growth plate chondrocytes

Question 32

How does the thyroid hormone enhance brain development?

Answer 32

Stimulates synapse and myelin fromation Stimulates growth of dendrites and axons on neurons Regulates neural crest cell migration and differentiation

Question 33

Describe the pathway of thyroid hormone secretion

Answer 33

Question 34

What is Graves Disease?

Answer 34

Autoimmune condition resulting in hyperthyroidism

Question 35

What causes Graves Disease?

Answer 35

Abnormal **thyroid-stimulating antibodies (TSI)** bind to TSH receptors on thyroid follicular cells Causes persistent thyroid hormone synthesis

Question 36

Symptoms of Graves Disease

Answer 36

High excitability Heat Intolerance Increased sweating Weight loss Muscle weakness Diarrhea Nervousness or other psychic disorders Fatigue with inability to sleep Tremor of the hands Exophthalmos

Question 37

What is myxedema?

Answer 37

Total lack of thyroid function in adults

Question 38

Symptoms of myxedema

Answer 38

Swelling of face (due to hyaluronic acid and chondroitin sulfate causing interstitial fluid accumulation) Husky and Slow voice Blood cholesterol increases and atherosclerosis develops

Question 39

How does iodine deficiency cause goiter development?

Answer 39

Thyroglobulin formed but no thyroid hormones TSH secretion remains uninhibited enabling hyperplasia and growth of thyroid gland

Question 40

What is Thyroiditis/Hashimoto disease?

Answer 40

Antibody develops against thyroglobin which destroys thyroid cells Causes hypothyroidism

Question 41

What is Cretinism?

Answer 41

Stunted physical and mental growth due to untreated congenital deficiency of thyroid hormones

Question 42

What causes cretinism?

Answer 42

Lack of thyroid hormone or iodine during prenatal, neonatal or childhood stages

Question 43

Effects of cretinism

Answer 43

Sluggish movement Retarded physical and mental growth Obese, stocky, short appearance Tongue becomes large