67 - Thyroid Hormones I

1. Thyroxine (T4)

2. Triiodothyronine (T3)

What 2 hormones does the thyroid gland produce?

1. Increase rate of basal oxygen use, basal metabolism, and rate of heat production

2. Modulate delivery of substrates and oxygen by cardiovascular and respiratory systems to sustain metabolic rate

What are the 2 main overall functions of T3 and T4?

1. energy need

2. calorie supply

3. environmental temperature

Thyroid hormone levels can change based on what 3 things?

1. tyrosines (2)

2. iodine

What 2 things do we need for thyroid hormone synthesis?

3,5,3',5'-tetraiodothyronine (thyroxine, T4)

What is the major thyroid hormone produced?

3,5,3'-triiodothyronine (T3)

What thyroid hormone is secreted in a lesser quantity and is a biologically active hormone?

reverse T3 (3,3',5'-triiodothyronine)

What is the alternative product of T4/inactive form of T3?

when less thyroid hormone action needed

When is reverse T3 (3,3',5'-triiodothyronine) produced?

single-layered circular follicles

What structures are comprised of cuboidal endocrine cells, surrounded by a basement membrane?

thyroglobulin

The lumen of the thyroid gland follicle has colloid material. The hormones stored here are attached to what?

absorbed into follicular cells

What happens to colloidal thyroid hormone under stimulation?

calcitonin

What do parafollicular cells (C cells) secrete?

iodine

For thyroid hormone synthesis, we need large amounts of what?

iodide

Iodine is supplied by the diet via what?

both

Is the synthesis of hormones intracellular, extracellular, or both?

extracellularly in follicular lumen

Where are thyroid hormones stored until stimulated to be secreted?

T4

What is the major hormones secreted by the thyroid gland?

T3

What is the biologically active thyroid hormone?

1. Need to synthesize protein molecule called thyroglobulin

2. Iodide transported from blood into follicular epithelial cells via 2 Na+/1 I- symporter

3. Iodide (I-) transported across apical membrane into colloid of follicle via pendrin and oxidized to iodine (I2)

4. Iodine incorporated into tyrosines at specific sites in thyroglobulin via thyroid peroxidase

5. Coupling reaction - results in MIT (monoiodotyrosine) and DIT (diiodotyrosine)

6. Two DIT molecules = T4

7. One DIT + one MIT = T3

What are the steps for synthesis of thyroid hormones?

via 2 Na+/1 I- symporter

How is iodide transported from blood into follicular epithelial cells?

via pendrin

How is iodide transported across the apical membrane into the colloid of the follicle?

T4

What do 2 diiodotyrosine (DIT) result in?

T3

What does 1 diiodotyrosine (DIT) and 1 monoiodotyrosine (MIT) result in?

10:1

What is the ratio of T4:T3 within the thyroid gland?

caused by ingestion of a large amount of iodine - lasts about 10 days

What causes the transient reduction in thyroid hormone levels, associated with the Wolff-Chaikoff effect?

activity of 2Na/ 1 I pump

What will excessively high levels of dietary iodide suppress?

hyperthyroid

We can give high iodide short-term to cats with what condition?

stimulation for release of thyroid hormones

Thyroglobulin is stored in follicles as colloid until when?

thyroglobulin

With stimulation for the release of thyroid hormones, what is retrieved from the follicle lumen and brought into the endocrine cell via endocytosis?

cell membrane forms pseudopods

With stimulation for the release of thyroid hormones, thyroglobulin is retrieved from the follicle lumen and brought into the endocrine cell via endocytosis. How?

1. T3

2. T4

3. free MIT

4. free DIT

Lysosomes fuse with colloid droplet and release what 4 thing from thyroglobulin?

yes

Do T3 and T4 enter the blood?

T4

What thyroid hormone has a large circulating pool size and long half life?

T3

What thyroid hormone has a small circulating pool size and a short half life?

1. age

2. breed size

With dogs, what 2 factors can effect circulating concentrations of T3 and T4?

1. Thyroxine-binding globulin (TBG)

2. Transthyretin

3. Albumin

T3 and T4 circulate bound to what 3 proteins?

help to buffer against acute changes in thyroid gland function

What do proteins help do, in relation to T3 and T4 circulation?

hepatic failure

Failure of what organ can affect circulating thyroid hormones?

sick euthyroid syndrome

What syndrome decreases circulating thyroid hormones?

sick euthyroid syndrome

What condition is associated with circulating levels of T3 and T4 being abnormal, but thyroid gland not appearing abnormal?

rT3

Sick euthyroid syndrome results in low T3 and T4. What is high with this condition?

1. Fasting/starvation

2. Stress (cortisol)

3. Catabolic diseases - cancer

4. Hepatic diseases

5. Renal diseases

What are the 5 conditions that cause sick euthyroid syndrome?

1. Thyrotropin-releasing hormone (TRH) is released from hypothalamus

2. Causes release of thyroid-stimulating hormone (TSH) from anterior pituitary

3. Causes release of T4 and T3 from thyroid gland

4. Negative feedback of T4 and T3 to anterior pituitary and hypothalamus

What are the steps in thyroid gland regulation?

TRH

What is a tripeptide hormone?

TSH

What is a glycoprotein hormone with 2 subunits?

1. α-subunit

2. β-subunit

What are the 2 subunits of TSH?

1. iodide transport

2. endocytosis of colloid

3. release of T3 and T4 from thyroid gland

What are the 3 things TSH stimulates?

TSH

What is there sustained exposure to by the thyroid gland?