L19: Thyroid Function Flashcards by Aefre Mak

What is the thyroid gland?

Part of the endocrine system, responsible for producing hormones that regulate metabolism

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Location of the thyroid

Anterior neck, inferior to larynx, surrounding the trachea

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Hormone in the hypothalamus & anterior pituitary that affects the thyroid

Hypothalamus: TRH
Anterior pituitary: TSH

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Where does the thyroid isthmus lie?

Halfway between the thyroid cartilage (adam’s apple) & the sternal notch

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What is the thyroid composed of?

Composed of hollow follicles (thyroid follicles)

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What is the thyroid lined by?

Epithelial cells (principal cells): responsible for the synthesis of thyroid hormones (T3 & thyroxine)

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What does the thyroid produce?

Thyroxine (T4)
Triiodothyronine (T3)

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What is the thyroid filled with?

Thyroid colloid: stores T3 & T4 prior to secretion

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2 types of C cells in the thyroid

1) Clear cells
2) Parafollicular cells

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Function of C cells

Responsble for the synthesis & secretion of calcitonin (regulates Ca2+ homeostasis)

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What hormones are synthesized by the thyroid gland?

T3 and T4

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Main function of the thyroid

Control the speed of metabolism

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How do T3 and T4 differ in terms of potency and activity?

T3 is more potent and shorter acting
T4 is less potent and longer lasting

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How does TSH affect the thyroid gland?

It stimulates T3/T4 production and thyroid growth

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What are amino acid hormones derived from?

Tyrosine

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How does tyrosine make T4?

Tyrosine + iodine -> T4

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What happens to T4 when it reaches target tissues?

It is de-iodinated to T3

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What physiological actions do thyroid hormones have on the body’s metabolism?

Increased oxygen consumption and increased heat production

What role does iodine play in the synthesis of thyroid hormones?

It is essential for the iodination of tyrosine residues on thyroglobulin

What is the role of T4 in the body?

Serving as a precursor to T3

How is the secretion of thyroid hormones regulated by the hypothalamus and pituitary gland?

TRH stimulates TSH release, which stimulates thyroid hormone production

What autoimmune condition is associated with hypothyroidism, and how does it affect the thyroid gland?

Hashimoto’s thyroiditis, causing thyroid destruction

Describe Hashimoto’s thyroiditis

1 in 1000 occurence
- TSH is increased as pituitary tries to compensate for decreased T4
- Treated with T3/T4 replacement

What are the symptoms of hypothyroidism?

Low metabolic rate
Weight gain,
Cold sensitivity
Deficient growth
Slow mental processes

How does hypothyroidism work?

Lack of T3/T4 means no inhibition of TSH release=elevated levels - TSH stimulates thyroid growth - Thyroid goitres form

How does the body respond to low levels of T3 and T4 in terms of TSH secretion?

TSH secretion increases due to a lack of inhibition

Where are thyroid hormone receptors (THRs) located in?

Nucleus

What do THRs require for transcription activation?

Heterodimer partner (RXR)

Physiological actions of thyroid hormones

1) Increased basal metabolic rate (BMR) & heat production 2) Enhance cardiovascular function 3) Stimulate NS activity 4) Regulate growth & development

Thyroid hormones effects on metabolism in the: 1) Liver 2) Muscles 3) Heart 4) Red Blood Cells

1) Increase gluconeogenesis & glycogenolysis 2) Increase protein catabolism 3) Increase cardiac activity 4) Stimulates production for better O2 transport

How is thyroid hormone secretion regulated?

TRH from hypothalamus stimulates TSH release from pituitary - TSH stimulates production & release of T3 & T4 from thyroid

What % of T4 in plasma is bound to Thyroid Binding Globulin (TBG)?

~99%

What is the HPT axis?

Hypothalamus-Pituitary-Thyroid Axis

2 types of thyroid gland disorders

1) Hyper thyroid gland 2) Hypo thyroid gland

What can cause congenital hypothyroidism, and what are its effects on development?

Thyroid failure from birth causing developmental abnormalities if untreated

Define cretinism

Abnormal mental/physical disorder resulting from deficiency of thyroid hormone in fetal/early life

What lab values indicate hypothyroidism?

Elevated TSH, decreased T4

What is the relationship between TRH and TSH in the regulation of thyroid hormone production?

TRH stimulates TSH release, increasing thyroid hormone production

What are the potential consequences of a lack of iodide in the diet regarding thyroid function?

Goiter and hypothyroidism

What is Graves' disease and what causes it?

Hyperthyroidism caused by thyroid-stimulating antibodies

What treatments are available for hyperthyroidism?

- Carbimazole or propylthiouracil - Surgery or radioactive iodine, and beta-blockers

How does carbimazole & propylthiouracil treat hyperthyroidism?

Inhibit thyroid peroxidase & decrease thyroid hormone synthesis

How does propylthiouracil treat hyperthyroidism?

Prevents conversion of T4 to T3

How does the structure of the thyroid gland facilitate its function?

he gland contains follicles arranged to maximize hormone production and storage capacity