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SFIV Test 2 > Thyroid Hormones: Synthesis and Regulation > Flashcards

Thyroid Hormones: Synthesis and Regulation Flashcards

1
Q

What is a goiter?

A

Enlarged thyroid

Does not describe the functional status of the gland

May be diffuse or nodular

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2
Q

What is the functional unit of the thyroid gland?

A

Follicle

A ductless alveolar structure surrounded by capillaries

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3
Q

What is thyroglobulin?

A

Glycoprotein where thyroid hormones are formed and stored

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4
Q

What are C cells?

A

Parafollicular cells located in the stroma surrounding the follicles

Produce Calcitonin

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5
Q

What are the secretory products of thyroid follicles?

A

Iodothryonines:

Thyroxine (T4)

Triiodothyronine (T3)

Reverse (rT3)

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6
Q

What is unique about thyroid hormones?

A

Only hormones to have iodine as a component

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7
Q

What is iodide trapping?

A

Transport system by which the thyroid gland absorbes iodine

Uses an Na/I symporter to move I against its concentration gradient

Efficiency is expressed in terms of the Thyroid/Serum Iodine ratio

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8
Q

What is the function of pendrin?

A

Iodide-chloride transporter responsible for transporting iodine into the follicular lumen for thyroid hormone formation

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9
Q

What is the function of thyroid peroxidase?

A

Oxidizes iodine as it enters the follicular lumen

Peroxide for this process is generated by thyroid oxidase

Also couples MIT and DIT to form T4 and T3

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10
Q

What occurs once iodine is oxidized?

A

Binds to tyrosyl residues in thryoglobulin forming monoiodotyrosine (MIT) and diiodotyrosine (DIT)

This process is called organification

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11
Q

What is coupled to form T4?

A

DIT + DIT

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12
Q

What is coupled to form T3?

A

MIT + DIT

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13
Q

What is the storage function of the MIT/DIT residues in thyroglobulin?

A

Serve as a reservoir for I

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14
Q

What is required for thyroid hormone release?

A

Proteolysis of thyroglobulin from the colloid via lysosomal proteases

T3 and T4 exit the basal surface

MIT and DIT remain in the cell and are deiodinated

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15
Q

What can occur in people deficient in microsomal iodotyrosine deiodinases?

A

Exhibit symptoms of hypothyroidism and have elevated urine MIT and DIT

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16
Q

What are the thyroid hormone binding proteins?

A

Thyroxine-Binding Globulin (TBG) - high binding affinity, T4>T3

Transthyretin - binds about 10% of T4, greater affinity for T4

Albumin -binds remainder

17
Q

What conditions can affect circulating levels of TBG?

A

Levels decline in malnutrition, liver and kidney disease

Hepatic TBG synthesis stimulated by estrogen

Salicylates decrease thyroid hormone binding to TBG

18
Q

What is the free fraction of thyroid hormones?

A

Hormone not bound to binding proteins

Available for tissue uptake

only about 0.03% of T4 and 0.3% of T3

19
Q

What are the circulatory half lives of the thyroid hormones?

A

T4 - 6-7 days

T3 - one day

20
Q

What are the major sources of T4 and T3?

A

All circulating T4 comes from the thyroid

Major sources of T3 are peripheral metabolism of T4 by the liver and kidneys

21
Q

What is the function of iodothyronine deiodinases?

A

Peripheral deiodinases that catalyze the removal of specific iodine moieties

Maintain the circulating pool of T3

Local regulation allows specific target tissue to be independently regulated

22
Q

Why can T4 be taken orally?

A

Sufficiently resistant to intestinal and hepatic degradation

23
Q

What is the most important stimulator of thyroid gland growth and function?

A

Thyroid stimulating hormone (TSH) or thyrotropin

TSH secrtion is stimulated by thyrotropin releasing hormone (TRH)

Inhibited by T3 and T4

24
Q

What is the pattern of secretion for TSH?

A

Pulsatile pattern and exhibits a circadian pattern

25
Q

What is the mechanism of action for TSH?

A

Gs protein that activates cAMP signaling pathway

26
Q

What are the actions of TSH?

A

Immediate - endocytocis and proteolysis of the colloid, releasing thyroid hormone

Increased expression of genes for thyroglobulin, thyroid peroxisdase and the NIS occur after a delay of hours-days

Chronic TSH stimulation promotes gland growth and capillary proliferation

27
Q

What is the regulatory effect of T3 and T4?

A

Exert negative feedback to both the pituitary and hypothalamus

However, the principal regulatory effect is at the pituitary

28
Q

Describe the autoregulation of the thyroid gland

A

Thyroid gland has an intrinsic abililty to regulate thyroid hormone production in response to iodide availability that is independent of TSH

29
Q

What is the Wolff-Chaikoff effect?

A

Ingestion of excessive amounts of iodine suppresses thyroid hormone synthesis

After a period of days, an escape mechanism is activated and thyroid hormone secretion resumes

Clinical: pharmacological doses of iodide, in the presence of anti-thyroid drugs, can be used to treat acute thyrotoxicosis

SFIV Test 2 (13 decks)

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