Thyroid Gland and Its Functions

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Questions and Answers

What is the main function of thyroid metabolic hormones T4 and T3?

To stimulate O2 consumption by body cells and regulate lipids and carbohydrate metabolism (correct)

To regulate body temperature

To produce calcitonin for calcium hemostasis

To produce thyroglobulin for protein synthesis

What is the percentage of carbohydrate in thyroglobulin by weight?

10% (correct)

20%

15%

What is the result of hypothyroidism in adults?

Mental and physical slowing, and poor resistance to cold (correct)

Body wasting and nervousness

Severe mental retardation and dwarfism

Excess heat production and tremor

What is the name of the hormone that stimulates the secretion of thyroid metabolic hormones?

Thyroid-stimulating hormone (TSH) Signup and view all the answers

Where does the synthesis of thyroglobulin take place?

In the thyrocytes Signup and view all the answers

What is the role of thyroid peroxidases in thyroid hormone production?

To mediate oxidation and reaction of iodine with thyroglobulin Signup and view all the answers

What is the biological active form of thyroid hormone?

T3 (Triiodothyronine) Signup and view all the answers

What is the function of the colloid in thyroid hormone production?

To store thyroid hormone Signup and view all the answers

What is the name of the amino acid from which thyroid hormones are synthesized?

Tyrosine Signup and view all the answers

What is the result of thyroid dysfunctions in fetal and neonatal life?

Absence or hypofunction causing severe mental retardation and dwarfism Signup and view all the answers

How is thyroid hormone released from thyroglobulin?

By lysosomal cleavage Signup and view all the answers

What is the first step in the synthesis of thyroid hormone?

Iodination of tyrosine to give MIT Signup and view all the answers

What is the percentage of T4 in the thyroid follicle?

35% Signup and view all the answers

What is the primary mechanism of thyroid hormone regulation?

Negative feedback on pituitary TSH secretion Signup and view all the answers

What is the function of the bound form of thyroid hormones in plasma?

Acts as a reservoir when needed Signup and view all the answers

What is the primary site of T4 deiodination into T3?

Liver, Kidneys, and other tissues Signup and view all the answers

What is the percentage of T4 that is free in plasma?

0.02% Signup and view all the answers

Which of the following plasma proteins has the highest affinity for T4?

Thyroxine binding globulin (TBG) Signup and view all the answers

Study Notes

Thyroid Gland

Location: Butterfly-shaped in the front of the neck
Size: The largest endocrine gland in the body
Secretions: Metabolic hormones (T4 and T3) and calcitonin (for calcium hemostasis)

Thyroid Metabolic Hormones (T4 and T3)

Function: • Stimulate O2 consumption by body cells • Regulate lipids and carbohydrate metabolism • Influence body mass and mentation

Thyroid Dysfunctions

In fetal and neonatal life: Absence or hypofunction causes severe mental retardation and dwarfism
In adult life: • Hypothyroidism: Mental and physical slowing, poor resistance to cold (cold intolerance) • Hyperthyroidism: Body wasting, nervousness, tachycardia, tremor, excess heat production

Thyroid Hormone Synthesis

Steps: • Transport of iodine into the colloid of thyroid follicles • Iodine oxidation in the colloid • Tyrosine iodination • Endocytosis of thyroglobulin-thyroid hormone into thyrocytes • Thyroid hormone release by cleavage in lysosomes
T4 is the main secreted hormone, while T3 is the biologically active form
T4 is converted to T3 in peripheral tissues by deiodination

Thyroglobulin

Structure: Glycoprotein containing 10% carbohydrate by weight and 123 tyrosine residues
Synthesis: In the thyrocytes
Secretion: By exocytosis into the colloid
Thyroid peroxidases mediate oxidation and reaction of iodine with thyroglobulin
The produced thyroid hormone remains part of thyroglobulin until needed

Thyroid Peroxidases

Involved in iodination and coupling of iodinated tyrosine residues

Synthetic Steps in Thyroid Hormone Production

First step: Iodination of tyrosine to give MIT (monoiodotyrosine)
Next step: Iodination of MIT at C5 to give DIT (diiodotyrosine)
Two possibilities then occur: • Two DIT undergo oxidative condensation to form T4 • One DIT condenses with one MIT to form T3 (major) and reverse T3 (RT3 – trace)

Distribution and Secretion

% Distribution in the thyroid follicle: • T4: 35% • T3: 7% • MIT: 3% • DIT: 33%
Secretion rate: • T4: 80 μg/day • T3: 4 μg/day • RT3: 2 μg/day • DIT & MIT: null

Plasma Levels and Binding

Plasma levels: T4 = 8 μg/dL, T3 = 0.15 μg/dL
Solubility: Both are lipophilic
In plasma: Present as bound form (bound to plasma protein) + free form (biologically active)
Free hormone is biologically active and exerts an inhibitory feedback on pituitary TSH secretion
Plasma proteins that bind thyroid hormones: • Albumin • Prealbumin (Transthyretin) • Thyroxine-binding globulin (TBG)

Metabolic Pathway

T4 deiodinated into T3 (catabolism and activation)
Site of metabolism: Liver, kidneys, and other tissues
Metabolism of T4: 1/3 to T3, 45% to RT3
T3 supply: 13% secreted by thyroid gland, 87% by deiodination

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Description

Learn about the thyroid gland, its location, size, secretions, and functions, as well as thyroid dysfunctions and their effects on fetal and neonatal life.