Lesson 2: Hormones and Regulation, thyroid glands/hormones

Describe the basic structure of the thyroid glands?

The thyroid gland consists of two lobes that are located on either side of the trachea just below the larynx.

The thyroid are richly supplied with blood from a branch of what artery?

Common Carotid Artery

What are the three parts of the thyroid gland?

Thyroid follicles

Thyroid Cells

C-Cells

What does the thyroid follicle consist of?

consists of the colloid

What is the function of the thyroid cells?

Production of thyroxine (T4) and tri-iodotyrosine (T3)

What is the function of C-cells?

Production of calcitonin

All hormones produced by thyroid the are what type of hormone?

Amine hormones

What are thyroid hormones made up of?

Tyrosine and iodine

Thyroid hormone is the exception to what rule?

The are an amine hormone that acts like a lipid soluble hormone.

How is iodine obtained since it is not produced in the body?

From the diet and is absorbed in the duodenum then into the blood circulation

What is the active process in which iodine is taken up by the thyroid follicular cells?

iodine trapping

The thyroid is the only organ that actively concentrates what from the bloodstream?

Iodine

How is iodine transported into the thyroid cells?

Sodium-iodine symporter (NIS)

What are the two components that are essential for the synthesis of thyroid hormones?

Iodine and tyrosine

Describe the steps of thyroid hormone synthesis?

1. NIS takes iodide from circulation into thyroid cell

2. TGB being synthesized and secreted into the colloid

3. Iodine is oxidized by peroxidases into iodine then pushed it into the colloid

4. TGB binds to the iodine in different degrees (T4 or T3 but in a big line)

5. Iodinated thyroglobulin is broken down in thyroid cells by lysosomal enzymes from which T3 and T4 are the products.

What is the function of TSH to the thyroid gland?

Stimulates iodine uptake

Stimulates iodination of tyrosine

Growth of thyroid cells

When is TSH stimulated?

Prolonged exposure to cold

Describe the negative feedback loop of when an animal is cold?

1. lower body temp causes hypothalamus to secrele TRH

2. TRH stimulates the anterior pituitary to release TSH

3. TSH stimulates follicular cells of the thyroid to produce TH.

4. TH stimulates target cells to increase metabolic activities such as increase body temp.

5. When body temp increases, TH will them block the TRH receptors on the AP.

What hormone is more common T4 or T3?

T4

What proteins do thyroid hormones bind to?

Thyroxine Binding globulin (TBG) and Albumine (ALB)

How does the binding to a protein effect the half life of thyroid hormones?

Longer half life than unbound thyroid hormones.

Why is having a longer half-life important for thyroid hormones?

Longer storage availability circulating in the blood

Where are thyroid hormones stored?

In circulation and in follicles

What happens once T4 reach the target cell?

T4 enters the cell and is broken down to T3 by deiodinase creating T3 and rT3 an inactive form.

Why does T4 need to be broken down into T3 in the target cell?

T3 is 7x more potent than T4

What explains the long half life of thyroid hormones?

Binding to plasma proteins

What happens to T3 in the nucleus?

T3 binds to a nuclear receptor and to DNA to activate gene transcription. Creating new intracellular proteins form.

How does T3 increase cell viability?

T3 will make Mitocondria bigger and replicate through the non-genomic pathway, this causes more ATP production.

What are the two main systems that are effected by the new proteins synthesized by T3?

Cardiovascular and Metabolism

How does the newly synthesized proteins effect the cardiovascular system?

Increase CO, HR, HS, and respiration

What are the main effects of thyroid hormones on the general metabolism?

Increase size and # of mitochondria

Increase BMR

Generates heat through oxidation

Increases Na/Kase pump activity

Increases metabolic heat produced

What is the main effect of thyroid hormones in the carb metabolism?

Increased glucose absorption and utilization

What is the main effect of thyroid hormones in the lipid metabolism?

Stimulates lipolysis

increases bile secretion and fat absorption

lowers blood cholesterol

What is the main effect of thyroid hormones on protein metabolism?

Growth depends on thyroid hormones

Increase in protein synthesis/cell division

in excess it destroys proteins

What is the main effect of thyroid hormones on the nervous system?

Essential for the development of CNS in fetuses/newborns

Excitatory effect of CNS

Enhances the responsiveness of the sympathetic target tissues

What is the main effect of thyroid hormones on the GI tract?

Increases appetite

Increases motility and secretion

Lack of thyroid hormones can cause constipation

What is the main effect of thyroid hormones on the skin and hair follicle?

Prolongs the duration of anagen

Inhibits the synethesis of hyaluronic acid, fibronectin, and collagen

Describe the Hair growth cycle.

1. Kenogen= no hair in follicule

2. Early anagen= start of hair growth

3. Late anagen= hair growth reaches its peak

4. Catagen= hair growth slows down

5. Telogen= hair stops growing

6. Exogen= hair falls out of follicle

How do thyroid hormone affect the respiratory system?

Thyroid hormones increases oxygen utilization and CO2 production leading to an increase respiration rate and depth

What are the two main dysfunction of thyroid levels?

Hyperthyroidism

Hypothyroidism

What is hyperthyroidism?

Occurs when the thyroid gland overproduces thyroid hormones

What is hypothyroidism?

Occurs when the thyroid gland produces insufficient amounts of thyroid hormones.

How does the abnormal level of thyroid hormones affect the normal physiology of metabolism in the body?

Hypothyroidism decreases metabolic rate, leading to weight gain and cold intolerance

Hyperthyroidism increases metabolic rate, leading to weight loss and heat intolerance

What are some effects of functional tumors on the thyroid causing hyperthyroidism?

Hyperactivity

Mated hair or alopecia

Weight loss due to increased proteolysis/lipolysis

Polyphagia

Vomiting and Diarrhea

Tachycardia

Panting/coughing

What are the effects/clinical signs of hypothyroidism?

insufficient thyroid hormones/TSH secretion

Obesity and Lethargy

Bradycardia/ arrhythmias

Dry, dull hair, alopecia

seborrhea

myxedema

hyperpigmentation

How does hypothyroidism effect the cardiovascular system?

reduces cardiac function by decreasing beta-adrenergic receptors and rate of Ca+ uptake

What is seborrhea?

Abnormal sebum production and or a defect in keratinization

What is Myxedema?

Excessive hyaluronic acid accumulates (sad face)

What is the main drug that is used to treat hyperthyroidism?

Methinmazole

What is the main drug that is used to treat hypothyroidism?

Levothyroxine

What is the mode of action for Methinmazole?

Blocks production of substrate for Thyroxine peroxidase leading to a decrease incorporation of iodine into tyrosine molecules.

What is the mode of action for Levothyroxine?

Synthetic hormone (T4) that restores normal levels and reverses clinical signs.

What happens to the release of TSH when T4 levels are excessively high?

TSH decreases.