E3 - Thyroid Gland

Question 1

Thyroid Gland Histology

Answer 1

Made of follicles
– Single layer of follicular epithelial cells surrounding a lumen full of proteinaceous colloid.
– Synthesize thyroid hormone

Question 2

Thyroid hormone synthesis

Answer 2

1. Iodine entry and concentration in thyroid follicular cells
2. Thyroid hormone synthesis/storage in the colloid
3. Release of thyroid hormone into the blood

Question 3

___ regulates NIS gene expression, prolongs NIS half- life and mobilizes NIS to cell membrane

Answer 3

TSH

Question 4

TSH regulates rate of organification and coupling by stimulating ___ gene expression

Answer 4

TPO

Question 5

Thyroid hormone secretion into blood

Answer 5

1. Endocytosis of colloid
2. Proteolysis releases T3/4
3. Deiodination releases I from MIT/DIT

Question 6

NIS

Answer 6

(Iodide transporter)

Cotransports Na+ and I- across basolateral membrane into cell

Question 7

Thyroid Peroxidase (TPO)

Answer 7

Catalyzes formation of thyroid hormone within thyroglobulin protein sequence through oxidation and organification of iodide and coupling of iodinated tyrosine residues within the protein

Question 8

Thyroglobulin

Answer 8

A glycoprotein that constitutes ~50% of thyroid cell protein

Question 9

Thyroid hormones (primarilyT3) are important for early growth and development.

Answer 9

-CNS: cognition and mood
-Sexual development: timing of puberty, menses
-Linear growth (modulate GH)

Question 10

Hypothyroidism Tx

Answer 10

Synthetic thyroid hormone replacement
– Levothyroxine (synthetic T4)

Question 11

Hyperthyroidism Tx

Answer 11

Medication to decrease thyroid hormone synthesis
– Methimazole, PTU (Propylthiouracil): Inhibit TPO

Question 12

• ___ of T4 is secreted by the thyroid gland ~ 100nmol daily
• ___ of T3 is secreted by the thyroid gland ~ 5nmol daily
• ___ of T3 is formed from peripheral conversion of T4 to T3

Answer 12

• 100%
• 20%
• 80%

Question 13

5’ deoidinase in peripheral cells is inhibited by:

Answer 13

-Fasting ≥ 2 days
-Severe illness
-Glucocorticoids
-Certain drugs

Question 14

Many cells express a 5’ de-iodinase that ____ the 5’ I of T4 to convert it to T3

Answer 14

Removes

Question 15

Thyroid hormones are ___ in water, and are ____ bound to plasma proteins to facilitate their distribution through the blood

Answer 15

-poorly soluble
-reversibly

Question 16

Most thyroid hormones are bound to ___

Answer 16

TBG

Question 17

– T3 half-life is ____
– T4 half-life is ____

Answer 17

-1 day
-1 week

Question 18

Changing TBG levels changes ____ T4 and T3 concentrations in blood but ___ levels remain stable

Answer 18

-Changes total
-Free remains same

Question 19

___ has 5-10x more biological activity than ___ on the TR

Answer 19

T3 has 5-10x more biological activity than T4 on the TR

Question 20

Thyroid hormone receptor (TR) is a ____ receptor

Answer 20

Nuclear

T3 binds to the ligand-binding
domain, activating transcription factors that regulate gene expression

Question 21

Iodine sources for thyroid hormones

Answer 21

1. Dietary Iodine
2. Plasma Iodide
3. Pharm (meds/CT contrast)

Question 22

The thyroid gland ____ hormone production in response to prevent thyroid hormone excess or deficiency due to acute changes iodine availability

Answer 22

Autoregulates

“Wolff-Chaikoff” effect