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ENDO > Thyroid Physiology 176 > Flashcards

Thyroid Physiology 176 Flashcards

1
Q

What is the functional unit of the thyroid gland?

A

the follicle

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2
Q

What are thyroid follicles filled with?

A

thyroglobulin

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3
Q

What is the general structure of TSH? How is it related to the structure of TSH, FSH, LH, and hCG?

A

it has an alpha and beta subunit

the alpha subunit is shared by all of these hormones

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4
Q

What are the three negative regulators of the hypothalamic-pituitary-thyroid axis?

A

T3 (negative feedback loop), somatostatin, and dopamine

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5
Q

What type of receptor does TSH bind to?

A

G-protein coupled receptor

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6
Q

What are the immediate downstream effects of TSH binding to the TSH receptor?

A

activates a Gs-alpha subunit –> activates adenylyl cyclase –> increases cAMP –> phosophorylates PKA –> activates targets in the nucleus

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7
Q

How are T3 and T4 synthesized?

A

iodine is transported into the cell (basolateral side) and is conjugated to tyrosyl residues in the presence of hydrogen peroxide

the iodotyrosines are coupled to either T3 or T4 (that enters the cell from thyroglobulins) and then released into the bloodstream

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8
Q

How does thyroglobulin contribute to thyroid hormone synthesis?

A

thyroglobulin carries T3, T4, and iodotyrosines and internalized into follicular cells by micro- and macropinocytosis and digested by lysosomes

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9
Q

For the most part, thyroid hormones circulate in their (bound/free) form.

A

For the most part, thyroid hormones circulate in their bound form.

  • less than 1% circulates as free hormone*
  • mostly bound to albumin, thyroxine-binding globulin, and transthyretin*
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10
Q

How is T4 metabolized into an active form?

A

metabolized to T3 by intracellular 5’-monodeiodination

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11
Q

What is the action of thyroid hormone in the nucleus?

A

it regulates gene transcription (either upregulates or downregulates it) in combination with accessory proteins, corepressors, and coactivators

there are also soem non-genomic targets

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12
Q

What is the relationship between TSH and FT4 levels?

A

inverse log-linear relationship

a small change in free T4 corresponds to an exponential change in TSH

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13
Q

How are T4 and T3 levels measured?

A

radioimmunoassay or immunometric methods

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14
Q

What is the clinical use of thyroglobulin measurements?

A

used as a marker in follow up of differentiated thyroid carcinoma following thyroidectomy/radioablation

normally circulates at low concentrations, but after total thyroidectomy/radioablation the levels should be undetectable

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15
Q

What is the utility of 131 iodine?

A
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16
Q

What is the utility of 123 iodine?

A

imaging and uptake studies

17
Q

What nutritional element is essential for synthesis of thyroid hormone? What is the daily recommended allowance of the micronutrient?

A

iodine

150 mcg daily

18
Q

How are thyroid hormones transported in the blood stream?

A

mostly bound to thyroxine binding globulin, transthyretin, and albumin

19
Q

What are key lab tests to evaluate thyroid function?

A

bloodwork: TSH, free T4, T3, thyroglobulin, calcitonin, thyroid antibodies (TPO and thyroid stimulating immunoglobulins)

radioiodine uptake and scan

20
Q

What are key tests to evaluate thyroid structure?

A

ultrasound, CT (especially when intrathoracic extension of large goiters)

21
Q

What is the most common cause of thyroid disease worldwide?

a) autoimmune thyroid disease
b) thyroid nodules
c) thyroid carcinomas
d) iodine deficiency

A

d) iodine deficiency

22
Q

Which of the following statements is true?

a) T4 is a prohormone that is converted to T3 within various target tissues
b) Reverse T3 is the active form of thyroid hormone
c) TPO catalyzes the conversion of T4 to T3
d) none of the above

A

a) T4 is a prohormone that is converted to T3 within various target tissues

reverse T3 is inactive, T4 is converted to T3 by deiodonases

23
Q

Antibodies agains the TSH receptor are associated with which thyroid disease?

A

Graves’ disease

24
Q

Antibodies agains the thyroperoxidase are associated with which thyroid disease?

A

Hashimoto’s thyroiditis

25
Q

What is calcitonin used as a tumor marker for?

A

medullary thyroid cancer

26
Q

What is the most likely diagnosis?

a) toxic adenoma
b) factitious thyrotoxicosis
c) thyroiditis
d) Graves’ disease

A

d) Graves’ disease

27
Q

What is the most likely diagnosis?

a) toxic adenoma
b) factitious thyrotoxicosis
c) thyroiditis
d) Graves’ disease

A

a) toxic adenoma

ENDO (19 decks)

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