The Thyroid Hormone Flashcards by Emma Strang

Where are hormones produced?

Endocrine glands

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Where are hormones secreted into?

Bloodstream

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Where are hormones aiming to reach?

Target cells

Act on receptors

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What kinds of effects can hormones have?

Rapid- Adrenaline (Epinephrine, target the heart)- increase in heart rate
Slow in onset and longer lasting- Androgens- build up of muscle tissue

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Give two examples of hormones?

Thyroid hormones

Adrenal gland hormones

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What does the hypothalamus act on?

The anterior pituitary

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What effect does the hypothalamus on the anterior pituitary gland?

Leads to increase or decrease in release of hormone?

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Where us the thyroid gland located and what’s it shape?

Located in the neck

Butterfly shaped

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What does the hypothalamus produce?

Thyrotrophin Releasing Hormone (TRH)- a tripeptide

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What does the anterior pituitary gland release in response to TRH?

Produces Thyroid Stimulating Hormone (TSH) or thyrotrophin a glycoprotein

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What thyroid hormones does the thyroid gland produce?

Tri-iodothyronine- T3

Tetra-iodothyronine- Thyroxine- T4

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Where are thyroid hormones synthesised and stored?

In the thyroid gland

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Where and when are thyroid hormones release?

Released when needed

Circulate in blood to their target tissues

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What do thyroid hormones control?

How much TRH and TSH is released

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What is the thyroid functional unit?

The follicle

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What is a follicle?

Single layer of epithelial cells around a cavity (follicle lumen) which is filled with colloid containing thyroglobulin

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What is thyroglobulin?

A large glycoprotein (contains tyrosine residues) is synthesised, glycosylated and then secreted into the lumen of the follicle, where iodination of the tyrosine residues occurs

What are the main steps of synthesis, storage and secretion of thyroid hormones?

Uptake of plasma iodide (iodide uptake, the main control point for hormone synthesis, is stimulated by TSH) by follicle cells
Oxidation of iodide to iodine (by peroxidase)
Iodine transferred to tyrosine residues in Thyroglobulin to give mono-iodinated and di-iodinated tyrosine (MIT and DIT)
Secretion of thyroid hormone

What are the 8 steps of the synthesis of a thyroid hormone?

Active uptake of iodide (I-)
Iodide may be discharged
Iodide uptake stimulated by TSH
Oxidation of iodide
Active transport of iodine
Formation of MIT and DIT
Uptake of the thyroglobulin
About 1% of stored colloid is removed each day. When the gland is very active this may rise to nearly 100% and colloid stores are depleted

Where are T3 and T3 synthesised?

The follicles of the thyroid gland

What is the shape of a follicle?

Sphere

Filled with glycoprotein, thyroglobulin which contains many tyrosine residues

What does TSH act on?

Receptors on the membrane of the thyroid follicle cells through the mechanism that involves cAMP

What does TSH acting on the membrane activate?

Hormone synthesis

What do iodinated tyrosines combine to form?

Tri-iodothyronine (T3) and tetra-iodothyronine (T4)

Where are T3 and T4 stored?

On thyroglobulin

What happens when blood levels of T3 and T4 are taken up by endocytosis into the follicle cells?

The endocytotic vesicles fuse with lysosomes and proteolytic enzymes act on thyroglobulin, releasing T3 and T4 enter the blood stream

How do T3 and T4 move in the blood?

>95% bound to plasma proteins

What are the nuclear receptors?

Intracellular

What is T4 converted to inside the cell?

T3 which acts on the receptor

What does the receptor associate with?

DNA

What does T3 acting on the receptor lead to?

mRNA and protein synthesis

What are thyroid hormones involved in?

Increasing the basal metabolic rate Increased heat production (oxidative metabolism) Increase glucose and amino acid uptake into cells Mitochondria- increase number and size mRNA and mRNA polymerase activity Protein synthesis All achieved through regulation of nuclear receptor activity

What is hypothyroidism for Hashimoto’s Thyroidtis?

Lack of a hormone

What does hypothyroidism in infancy for Hashimoto’s Thyroidtis show symptoms of?

Mental retardation | Dwarfism

What are symptoms of hypothyroidism in Hashimoto’s Thyroidtis?

``` Depressed metabolic rate Depressed appetite Depressed Cardiac ouput Tired Lethargic ```

How is hypothyroidism treated?

Giving T4- Thyroxine

What are the symptoms of hypothyroidism in Grave's disease?

``` Enlarged thyroid (goitre) Protruding eyes (exophthalmos) ```

How is Grave's Disease caused?

Antibody that mimics the effects of TSH is produced Leads to excess stimulation of the thyroid gland Causes increased release of T3 and T4 Excess of hormone Increased metabolic rate, appetite Sympathetic activity increased Nervous and motor activity increased

How is Grave's disease treated?

Surgery Irradiation with iodine 131 Iodides Drugs which inhibit hormone synthesis

What is Thioureylenes?

E.g. Propylthiouracil; Methimazole; Carbismazole Inhibit peroxidase conversion of iodide to iodine Reduces iodination of tyrosine residues in thyroglobulin Coupling of the iodinated tyrosine to form T3 and T4 Conversion of T4 to the active form T3

What do thyroid hormones promote the synthesis of?

Adrenoceptors

How can hyperthyroidism be treated?

Treated with beta adrenoceptor antagonists