Lesson 16: Thyroid gland

Shape and location of thyroid gland

Shaped like a butterfly and located just below larynx

How is the thyroid gland composed

Composed of left and right lateral lobes, one on each side of the trachea connected by isthmus

Endocrine axis of TSH

What is the functional unit of the thyroid gland and how is the structure

Thyroid follicles are the functional unit of the thyroid gland and its a spherical structure whose walls are made up of follicular cells / thyrocytes on a basement membrane

What happens when they are stimulated TSH

They change they shape from flattened to cuboid/cylindrical and synthesize:

• T4 = Thyroxine

• T3 = triiodothyronine

What other cells types do thyroid glands have and what is their function?

Parafollicular cells or C cells lie between follicles and produce the Hormone calcitonin, which helps regulate calcium homeostasis

What type of hormones are T3 and T4 and how are they formed?

Iodothyronine type hormones

Formed through coupling of iodinated tyrosine molecules

T3 and T4 characteristics?

T4:

• Thyroxine

• 90% of total thyroid hormone secretion

• Not active form

T3:

• Tiiodothyronine

• 10% of total tyhroid hormone secretion

• Biologically active form

What is required for the formation of thyroid hormones and how do we obtain it and how much is required for normal quantities of thyroid hormones?

Iodine

Uptake by diet and reuse of iodine in body

1mg/week is required for normal quantities of thyroxine hormone

Synthesis and storage of thyroid hormones

1. Iodide entrapment: Thyroid follicular traps iodide ions by active transport from blood to cytosol

2. Thyroglobulin synthesis: Follicular cells synthesize thyroglobulin (TGB) in the rough endoplasmic reticulum, which is released into the lumen of the follicle by exocytosis.

3. Oxidation of iodide to molecular iodine: that diffuses into the lumen of the follicle

4. Tyrosine iodination: iodine reacts with tyrosines forming sticky material that is the colloid

• T1 = monoiodinated tyrosine; T2 = tyrosine with 2 iodine ionś

5. Union of T1 and T2: 2 T2 molecules join to form T4 and one T1 and one T2 join to form T3

6. Pinocytosis and colloid digestion: Colloid droplets re-enter the follicular cells by pinocytosis and

   bind to lysosomes where TGB is degraded, releasing T3 and T4 molecules

7. Secretion thyroid hormones: : As T3 and T4 are fat-soluble, they diffuse through the plasma

   membrane. In the blood they travel bound to proteins

Release into blood and transport of thyroid hormones

1. T3 (10%) and T4 (90%) enter blood after stimulus

2. Circulate bound to plasma proteins (99%)

3. T4 has higher affinity for proteins → free fraction correspond mainly to T3

4. Free fraction t3 is the only that can access tissues

5. Fraction of T4 is like a reservoir of T3 in blood as it can be deiodinated and become T3

How is TSH regulated

By negative feedback produced by thyroid hormone levels

What are inhibitors for the secretion of thyroid hormones

Stress, Antithyroid drugs or food, iodine defficiency

What are the effects of T3 and T4

Increase body tempertaure

Increase blood glucose levels

Growth regulation

Increase heart rate

Which hormones regulate the homeostasis of calcium and phosphate and what type are they

Calciotropic hormones

Decrease calcium: calcitonine

increase calcium: Vitamin D, PTH

What type of hormone is Calcitonin and how is it produced

Polypeptidic Hormone secreted by C cells of thyroid glands

Which stimulus causes release of calcitonin and where does its effects occur mainly?

Hypercalcemia

Bone

What are its effects on bones

• decrease formation and activity of osteoclast

• Decrease bone resoprtion

• Simulates osteoblasts by increasing uptake of calcium and phosphate in bone

Where is PTH = parathyroid hormone produced

Produced in 4 parathyroid glands located on posterior surface of thyroid gland

What are the types of epithelial cells of parathyroid glands?

Main cells: majority, produce PTH

Oxyphil cells: unknown function

What is main stimulus of PTH

Hypocalcemia

Effects of PTH

Bone:

• induces an increase in osteoclast activity, increases bone resorption

• calcium and phosphate released into blood

Kidney:

• second hydoxylation vitamin D§

• Decrease calcium loss

• Increase phosphate loss

How is vitamin D3 (calcitrol) formed

Endogenous or through diet

Endogenous: UVB light in most basal layer of skin

When is vitamin D3 active

It must be hydroxylized in the liver and then kidney

Physiological action of Vitamin D3

Bone: Increase mineralization

Intestine: absorption of calcium from and from phosphate

Kidney: reabsorption calcium and phosphate

Response Hypocalcemia reactions

1. detected by Paraythyroid glands main cells and stimulates PTH secretion

2. PTH exerts hypercalcemic effects

3. PTH induces increase of 1,25-vitamin D

4. Promotes greater bone resorption → release calcium and phosphate to blood

5. PTH activity and renal hydroxylation of vitamin D will decrease as calcium level are normalized

Response hypercalcemic situations

1. Hypercalcemia deactivates action of PTH and Vitamin D

2. Hypercalcemia stimulates release of calcitonin

3. Calcitonin lower blood calcium levels and promote bone deposition