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Endocrinology > Physiology of the Thyroid Gland > Flashcards

Physiology of the Thyroid Gland Flashcards

1
Q

Where is the thyroid gland located?

A
  • anterior to the trachea and below the larynx (midline of the neck).
  • can feel an increase in size when the gland hypertrophies (normal weight= 15-20 grams).
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2
Q

Does the thyroid gland of a good blood supply?

A

YES, bc it releases hormones to the blood.

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3
Q

What is one of the largest endocrine organs?

A
  • thyroid gland
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4
Q

What does the thyroid gland secrete?

A
  • triiodothyronine (T3)= 7%
  • thyroxine (T4)= 93%
  • calcitonin
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5
Q

** Is T3 or T4 more potent?

A
  • T3 (4x more potent)

* however, it’s presence is much lower than T4

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6
Q

Into what is T4 converted in the blood?

A
  • T4 is converted to T3 in the blood
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7
Q

** What stimulates the thyroid gland to secrete T3/T4?

A
  • thyroid stimulating hormone (TSH)
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8
Q

** What gland modulates the metabolic rate?

A
  • thyroid gland
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9
Q

What will you see on histology of the thyroid?

A
  • follicles (contains T3/T4) and colloid/thyroglobulin
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10
Q

*** How much IODINE does the thyroid gland require per week to make thyroxine?

A
  • 1 mg

* iodized salt, fish and seafood are good sources

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11
Q

How is iodine metabolized?

A
  • oral route (PO) absorbed like chloride and excreted by the kidneys.
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12
Q

*** How is T4 synthesized? (see diagram)

A
  • thyroid cells synthesize and secrete thyroglobulin into the follicle (tyrosine amino acids).
  • oxidation of iodide ion by PEROXIDASE and HYDROGEN PEROXIDE.
  • iodination of tyrosine (organofication) via IODINASE
  • mono- di- triiotyrosine progression.
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13
Q

How is iodide added to thyroid hormones?

A
  • Iodide is taken into the thyroid cells (iodide trapping) and secreted into the lumen of the follicle (colloid), where it iodinates thyroid hormones.
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14
Q

How much thyroid hormone is stored in the thyroid gland?

A
  • 2-3 month supply
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15
Q

How is thyroid hormone released into the blood?

A
  • taken from the colloid, cleaved by the lysosomes, and released into the blood.
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16
Q

What happens to thyroxine (T4) in the blood?

A
  • binds to thyroxine-binding globulin (TBG)

* T3 has lower affinity for TBG

17
Q

What is the mechanism by which T3/T4 work?

A
  • cAMP second messenger system (bind to cell surface)
18
Q

** What are the effects of thyroid hormones?

A
  • increased PROTEIN SYNTHESIS
  • increased PROTEIN CATABOLISM
  • increased GROWTH
  • increased MENTAL EXCITEMENT
  • increase in SYSTEMIC ENDOCRINE ACTIVITY (menses, sweating…)
  • increased NUMBER OF MITOCHONDRIA (increases body temp; HEAT)
  • increased Na+/K+ ATPase at cellular membranes
  • increased FETAL BRAIN DEVELOPMENT
  • increased CARBS, FAT, and PROTEIN METABOLISM
  • increased NEED for VITAMINS
  • increased basal metabolic rate (BMR)
  • increased HEART RATE
  • increased IONOTROPY (contractility) of the MYOCARDIUM
  • increased GI MOTILITY
  • increased TREMORS
  • increased FREE FATTY ACIDS in PLASMA
  • decreased CHOLESTEROL levels in the PLASMA
  • decreased PHOSPHOLIPIDS
  • decreased TRIGLYCERIDES
  • decreased BODY WEIGHT
  • decreased sleep
  • remember T3 does this better than T4
19
Q

Is thyroid hormone needed for normal sexual function?

A

YES

20
Q

Does hyper or hypothyroidism lead to menstrual defects?

A
  • HYPOthyroidism
21
Q

What are the effects of TSH?

A

increased:

  • proteolysis of thyroglobulin.
  • iodide pump activity.
  • increased iodination of tyrosine.
  • increased size and activity of thyroid cells.
  • modulated by cAMP
22
Q

** How do we REGULATE/SUPPRESS thyroid hormone production? (TEGRITY)

A
  • thiocyante/perchlorate= decreased iodide trapping, not thyroglobulin synthesis
  • PTU (propylthiouracil)= decrases thyroid hormone formation.
  • iodide in high concentrations= decreases thyroid activity and size.
23
Q

What are some diseases of the thyroid?

A
  • HYPERthyroidism: Grave’s disease, thyrotoxicosis…
  • HYPOthyroidism:
  • toxic goiter= continues stimulation of the thyroid due to hormone synthesis breakdown.
  • myxedema= deficient hormone production.
24
Q

What are some diseases of the thyroid?

A
  • HYPERthyroidism: Grave’s disease, thyrotoxicosis (thyroid storm)
  • HYPOthyroidism (treat with hormone replacement).
  • toxic goiter= continuous stimulation of the thyroid due to hormone synthesis breakdown.
  • myxedema= deficient hormone production.
  • cretinism
25
Q

What is important to know about calcitonin?

A
  • secreted by parafollicular cells of the thyroid causing blood calcium to decrease.
  • remember calitonin TONES THE BONES
26
Q

Can thyroid dysfunction cause exophthalmos (bulging eyes)?

A
  • YES
27
Q

What causes TRH, and thus TSH to be released?

A
  • cold

- emotional stress

28
Q

What causes termination of TSH release?

A
  • increased levels of thyroid hormones in the blood (negative feedback).
29
Q

** Should you use PTU (propylthiouracil) before or after iodide, when attempting to suppress the thyroid gland? (TEST QUESTION)

A
  • BEFORE
30
Q

What should you check when evaluating a pt for HYPERthyroidism?

A
  • BMR/TSH and thyroid stimulating initiator levels.

* remember to prescribe PTU before Iodine to suppress it.

Endocrinology (18 decks)

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