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Chapter 19: Thyroid Gland

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What is the primary function of the thyroid gland?

To secrete thyroid hormones that maintain optimal metabolism

What is the consequence of thyroid gland dysfunction during fetal and neonatal life?

Mental retardation and dwarfism

What hormone regulates circulating levels of calcium?

Calcitonin

What is the molecular weight of thyroglobulin?

660 kDa

What is the percentage of carbohydrate in thyroglobulin by weight?

10%

How many tyrosine residues are normally incorporated into thyroid hormones?

4-8

What is the result of hyperthyroidism?

Body wasting, nervousness, tachycardia, tremor, and excess heat production

What hormone stimulates the secretion of thyroid-stimulating hormone (TSH)?

Thyrotropin-releasing hormone (TRH)

What is the primary function of thyroid peroxidase in the thyroid gland?

To iodinate thyroglobulin and generate reactive iodine species

What is the percentage of MIT in the normal human thyroid?

3%

What is the byproduct of the condensation of MIT with DIT?

T3

How long can humans survive without iodide in their diet before a decline in circulating thyroid hormone levels is seen?

2 months

What is the daily secretion rate of T4 by the human thyroid gland?

80 μg/day

What is the process by which colloid is internalized by thyrocytes?

Endocytosis

What is the percentage of DIT in the normal human thyroid?

33%

What is the function of lysosomes in thyroid hormone synthesis?

To degrade thyroid hormones

What percentage of circulating T4 is converted to T3 in adult humans?

One third

What is the daily conversion rate of T4 to T3?

27 µg/day

What percentage of circulating RT3 is secreted by the thyroid?

5%

What is the function of deiodination reactions in the body?

To catabolize thyroid hormones

What hormone increases TSH secretion?

TRH

What is the normal total plasma T4 level in adults?

8 μg/dL

What is the effect of T4 on TSH secretion?

Inhibition

Which protein has the largest capacity to bind T4?

Albumin

What is the immediate effect of TSH on the thyroid?

Increased iodide binding

What is the half-life of TBG?

5 days

What is the long-term effect of TSH stimulation on the thyroid?

Enlargement

What is the primary function of RT3?

Unknown

What is one of the widespread effects of thyroid hormones in the body?

Increased calorigenic action

Where do deiodination reactions occur?

In the liver, kidneys, and many other tissues

How do thyroid hormones enter cells?

By binding to TR in the nucleus

What is the daily conversion rate of T4 to RT3?

36 µg/day

What percentage of circulating T4 is converted to RT3 in adult humans?

45%

What percentage of circulating RT3 is secreted by the thyroid?

5%

What is the effect of T4 on TSH secretion?

It inhibits TSH secretion

What is the result of prolonged TSH stimulation on the thyroid?

The thyroid gland becomes detectably enlarged

What is the effect of thyroid hormones on oxygen consumption?

They increase oxygen consumption

What is the mechanism of action of thyroid hormones?

They bind to TR in the nuclei

What is the effect of T3 on hemoglobin?

It increases the dissociation of oxygen from hemoglobin

What is the effect of TSH on iodide binding?

It increases iodide binding

What is the effect of TSH on thyroid function?

It stimulates thyroid function

What is the effect of thyroid hormones on lipid metabolism?

They increase lipid metabolism

Study Notes

Thyroid Gland

  • The thyroid gland has two primary functions: secreting thyroid hormones to maintain optimal metabolism and secreting calcitonin to regulate calcium levels.
  • The thyroid gland is not essential for life, but its absence or hypofunction during fetal and neonatal life results in severe mental retardation and dwarfism.

Thyroid Hormone Synthesis

  • Thyroglobulin, a glycoprotein, is synthesized in thyroid cells and secreted into the colloid.
  • Thyroid peroxidase, a membrane-bound enzyme, oxidizes and reacts with iodide to form thyroid hormones within thyroglobulin.
  • The synthesis process involves the formation of monoiodotyrosine (MIT), diiodotyrosine (DIT), and finally, triiodothyronine (T3) and thyroxine (T4).

Regulation of Thyroid Secretion

  • Thyroid function is regulated primarily by pituitary TSH (thyroid-stimulating hormone), which is increased by hypothalamic TRH and inhibited by circulating free T4 and T3.

Effects of TSH on the Thyroid

  • TSH stimulates thyroid function, increases iodide binding, and induces the synthesis and secretion of thyroid hormones.
  • Long-term TSH treatment leads to thyroid enlargement (goiter).

Effects of Thyroid Hormones

  • Thyroid hormones stimulate O2 consumption, affect growth and development, regulate lipid metabolism, increase carbohydrate absorption, and influence red blood cell function.
  • T3 binds to thyroid receptors (TR) in nuclei, and T4 can also bind, but less avidly.

Thyroid Hormone Distribution

  • The normal human thyroid secretes approximately 80 μg/day of T4, 4 μg/day of T3, and 2 μg/day of RT3.
  • A portion of T4 is converted to T3 (27 μg/day) and RT3 (36 μg/day), and some is prepared for excretion as conjugates (17 μg/day).

Protein Binding

  • Thyroid hormones are relatively lipophilic and bind to proteins in plasma and tissues, with T4 and T3 being physiologically active in their free forms.
  • Albumin, transthyretin, and thyroxine-binding globulin (TBG) are the primary proteins that bind thyroid hormones.

Thyroid Gland

  • The thyroid gland has two primary functions: secreting thyroid hormones to maintain optimal metabolism and secreting calcitonin to regulate calcium levels.
  • The thyroid gland is not essential for life, but its absence or hypofunction during fetal and neonatal life results in severe mental retardation and dwarfism.

Thyroid Hormone Synthesis

  • Thyroglobulin, a glycoprotein, is synthesized in thyroid cells and secreted into the colloid.
  • Thyroid peroxidase, a membrane-bound enzyme, oxidizes and reacts with iodide to form thyroid hormones within thyroglobulin.
  • The synthesis process involves the formation of monoiodotyrosine (MIT), diiodotyrosine (DIT), and finally, triiodothyronine (T3) and thyroxine (T4).

Regulation of Thyroid Secretion

  • Thyroid function is regulated primarily by pituitary TSH (thyroid-stimulating hormone), which is increased by hypothalamic TRH and inhibited by circulating free T4 and T3.

Effects of TSH on the Thyroid

  • TSH stimulates thyroid function, increases iodide binding, and induces the synthesis and secretion of thyroid hormones.
  • Long-term TSH treatment leads to thyroid enlargement (goiter).

Effects of Thyroid Hormones

  • Thyroid hormones stimulate O2 consumption, affect growth and development, regulate lipid metabolism, increase carbohydrate absorption, and influence red blood cell function.
  • T3 binds to thyroid receptors (TR) in nuclei, and T4 can also bind, but less avidly.

Thyroid Hormone Distribution

  • The normal human thyroid secretes approximately 80 μg/day of T4, 4 μg/day of T3, and 2 μg/day of RT3.
  • A portion of T4 is converted to T3 (27 μg/day) and RT3 (36 μg/day), and some is prepared for excretion as conjugates (17 μg/day).

Protein Binding

  • Thyroid hormones are relatively lipophilic and bind to proteins in plasma and tissues, with T4 and T3 being physiologically active in their free forms.
  • Albumin, transthyretin, and thyroxine-binding globulin (TBG) are the primary proteins that bind thyroid hormones.

This quiz covers the thyroid gland, its functions, and the role of thyroid hormones in metabolism and body function. Learn about the importance of the thyroid gland in regulating bodily functions.

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