S7) The Thyroid Gland Flashcards Preview

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Briefly, describe the structure of the thyroid gland

2 lobes joined by isthmus


Describe the location of the thyroid gland

- Lies against and around front larynx and trachea

- Below thyroid cartilage 

- Isthmus extends from 2nd to 3rd rings of the trachea


Thyroid is first endocrine gland to develop.

In 5 steps, describe the embryological development of the thyroid gland

⇒ Appears as an epithelial proliferation in floor of pharynx at base of tongue

⇒ Descends as diverticulum through thyroglossal duct

Migrates downwards in front of hyoid bone

⇒ Remains connected to tongue by thyroglossal duct

⇒ Duct degenerates and detached thyroid continues to final position 


Describe the microscopic structure of the thyroid gland

Follicular cells arranged in spheres called thyroid follicles

- Follicles filled with extracellular colloid, a deposit of thyroglobulin


The thyroid and parathyroid are distinct glands. 

Describe how their cells vary in terms of hormone production


Describe how thyroid hormones are formed

2 tyrosine molecules are linked together with iodine at 3/4 positions on the aromatic rings 


What is the role of thyroglobulin in thyroid hormone formation?

Thryoglobulin acts as a scaffold on which thyroid hormones are formed 


What is thyroid peroxidase and what does it do?

Thyroid peroxidase is a membrane bound enzyme that regulates thyroid hormone synthesis


Thyroid peroxidase regulates 3 separate reactions involving iodide.

Identify these

Oxidation of iodide to iodine (requires H2O2)

- Addition of iodine to tyrosine acceptor residues on the protein thyroglobulin

Coupling of MIT/DIT to generate thyroid hormones within the thyroglobulin protein


How is dietary iodine absorbed in the thyroid gland?

- Iodine is oxidised to iodide before absorption in the small intestine

- Iodide (I-) is taken up from blood by thyroid epithelial cells via a sodium-iodide symport


Explain the following statement: most of T4 is converted to T3 outside of the thyroid

- 90% of thyroid hormone secreted is T4

- Most T4 is converted to T3 in the liver & kidneys 

- 80% of circulating T3 is derived from T4


Which thyroid hormone is more biologically active?

Biological activity of T3 is 4x that of T4


How are the thyroid hormones transported in the blood?

T3 & T4 are transported in blood bound to the protein thyroxine-binding globulin


Thyroid hormones effect virtually every cell in the body and have two interconnected responses.

What are these?

- Effects on cellular differentiation and development

- Effects on metabolic pathways 


Regulation of thyroid hormone secretion is via negative feedback.

Outline this


Describe the structure of thyroid stimulating hormone

Glycoprotein hormone composed of 2 non-covalently bound subunits:

- α subunit is also present in FSH and LH

- β subunit provides unique biological activity 


TSH is a trigger for thyroid hormone release.

Identify the seven processes stimulated sequentially

⇒ Iodide uptake

⇒ Iodide oxidation (iodine)

⇒ Thyroglobulin synthesis

⇒ Thyroglobulin iodination

⇒ Colloid pinocytosis into cell

⇒ Proteolysis of thyroglobulin

⇒ Cell metabolism & growth 


Describe the cellular mechanisms involved in the TSH induced signalling pathway for thyroid hormone secretion


Identify the three general effects of thyroid hormones on the body

- Increase in BMR and heat production

- Stimulation of metabolic pathways

- Sympathomimetic effects


How do thyroid hormones increase BMR and heat production?

- Increases number and size of mitochondria

- Stimulates synthesis of enzymes in respiratory chain


Describe two metabolic pathways stimulated by thyroid hormones 

Catabolic pathways:

- Lipid metabolism – lipolysis and β-oxidation

- Carbohydrate metabolism – insulin-dependent entry of glucose into cells, gluconeogenesis and glycogenolysis


What are the sympathomimetic effects of thyroid hormones?

Sympathomimetic effects – increases target cell response to catecholamines by increasing receptor number


Identify the tissue specific effects of thyroid hormones in terms of the following:

- Cardiovascular system

- Nervous system

CVS – increases heart’s responsiveness to catecholamines (increased cardiac output and peripheral vasodilation)

- Nervous system – myelination of nerves & development of neurones  


Describe the function and action of thyroid hormone receptors

- Function as hormone-activated transcription factors

- Act by modulating gene expression 


In four steps, outline the cell signalling pathway induced by thyroid hormones on intracellular receptors


Indicate the normal plasma levels of thyroid hormones as well as reasons for variations unrelated to thyroid disease


What is goitre?

Goitre is the enlargement of the thyroid gland due to its overstimulation

- It may accompany either hypo- or hyperthyroidism


What is hypothyroidism?

Hypothyroidism is an endocrine disorder wherein the thyroid gland is underactive and produces an insufficient amount of T3/T4


How does a patient with hypothyroidism present?

- Obesity

- Lethargy

- Intolerance to cold

- Bradycardia

- Dry skin

- Alopecia


What are the thyroid function test results for a patient with hypothyroidism?

- Low T3

- Low T4

- Elevated TSH