Lecture 20 - Thyroid Hormones

Question 1

How is iodine absorbed into the body and how much is needed?

L20 S4

Answer 1

50mg of iodine per year by way of iodide salts

Absorbed via sodium-iodide symporter (2:1) in basal membrane

Transported out of cell via pendrin, a chloride-iodide counter-transporter, in apical membrane

Can concentrate up to 30-250x

Question 2

How are thyroid hormones synthesized?

L20 S5

Answer 2

Iodine combined with tyrosine by peroxidase to produce:

• monoiodotyrosine (MIT)
• diiodotyrosine (DIT)

Recombination occurs to form:

• DIT + DIT = thyroxine (T4); more produced, less active
• DIT + MIT = triiodothyronine (T3); less produced, more active

These are all attached to thyroglobulin to be stored in colloid

Question 3

Describe thyroid hormone/iodine storage in the thyroid.

L20 S10

Answer 3

Thyroglobulin:

• secreted into follicle
• contains 70 tyrosine AAs
• stores about 30 thyroxine

Iodine:
-either nascent I2 or oxidized iodine I3- bind tyrosine of thyroglobulin

Question 4

How are thyroid hormones released and what enzymes are involved?

L20 S12

Answer 4

Apical surface of follicle cells form pinocytic vesicles containing colloid.

Lysosome fuses with vesicle to release T3 and T4 which are released into blood. where they are bound by thyroxine binding globulin (liver).

5’-iodinase converts T4 to T3

Deiodinase releases iodine from tyrosines for recycling

Question 5

What are the functions of thyroid hormones?

L20 S15

Answer 5

• increased gene transcription
• increased cellular metabolic activity
• effects growth
• increased cardiovascular activity
• excites CNS
• effects endocrine function

Question 6

What factors effect thyroid hormone secretion?

L20 S19

Answer 6

• TSH
• cAMP
• TRH
• cold or other neurogenic stimuli
• thyroid hormone feedback

TRH release from hypothalamus stimulates->
TSH from anterior pituitary stimulates ->
Follicular cell cAMP stimulates ->
Thyroxine secretion

Question 7

What is hyperthyroidism, what are its causes, what are its symptoms, and how is it treated?

L20 S22-23

Answer 7

High concentrations of circulating thyroxine and low TSH

Causes:

• Graves’ disease (IgG component bind TSH receptor)
• adenoma

Symptoms:

• high state of excitability
• intolerance to heat/increased sweating
• weight loss
• diarrhea
• weakness/fatigue

Treatment:

• removal of portion of thyroid
• radioactive iodine

Question 8

What is hypothyroidism, what are its causes, and what are its symptoms?

L20 S24

Answer 8

Causes:

• Hashimoto disease (autoimmune)
• endemic goiter
• cretinism

Symptoms:

• opposite of hyperthyroidism (decreased metabolism and related traits)
• myxedema
• atherosclerosis

Treatment:
-oral medications