Chapter 20: The Thyroid Gland

Flashcards about the thyroid gland, its functions, hormones, and regulation.

What are the two primary functions of the thyroid gland?

To secrete thyroid hormones for regulating metabolism, and to secrete calcitonin for regulating calcium levels.

What is the gross anatomy of the thyroid gland?

Butterfly-shaped gland straddling the trachea, connected by the thyroid isthmus, sometimes with a pyramidal lobe.

Describe the histology of the thyroid gland when inactive

Abundant colloid, large follicles, and flat cells lining them.

Describe the histology of the thyroid gland when active

Small follicles, cuboid or columnar cells, and reabsorption lacunae.

What are the primary hormones secreted by the thyroid?

Thyroxine (T4) and triiodothyronine (T3).

How is T3 generated at its site of action?

By deiodination of T4 in peripheral tissues.

Why is iodine an essential raw material in the thyroid?

Iodine is needed as an essential raw material for thyroid hormone synthesis.

What is the minimum daily iodine intake to maintain normal thyroid function in adults?

150 µg.

What is the role of the Na+/I- symporter (NIS) in thyrocytes?

It transports two Na+ ions and one I- ion into the cell.

How does TSH regulate NIS?

TSH induces both NIS expression and its retention in the basolateral membrane.

What is the role of pendrin in thyroid hormone synthesis?

Pendrin is a Cl–/I– exchanger that transports iodide across the apical membrane of thyrocytes to access the colloid.

What is organification in thyroid hormone synthesis?

The process where iodide is oxidized to iodine and incorporated into tyrosine residues of thyroglobulin.

What mediates the organification of iodide?

It is mediated by thyroid peroxidase, a membrane-bound enzyme in the thyrocyte apical membrane.

What are the four functions of thyrocytes?

To collect and transport iodine, synthesize thyroglobulin, fix iodine to thyroglobulin, and remove/secrete thyroid hormones into the circulation.

Name the iodinated compounds present during thyroid hormone synthesis.

monoiodotyrosine (MIT), diiodotyrosine (DIT), T4, and T3.

What happens to MIT and DIT that are not secreted?

They are deiodinated by a microsomal iodotyrosine deiodinase and recycled.

What are the normal plasma levels of T4 and T3 in adults?

Approximately 8 µg/dL for T4 and 0.15 µg/dL for T3.

What is the function of protein-binding in thyroid hormone transport?

To maintain a large pool of hormone that can be readily mobilized and to prevent excess uptake by first cells encountered.

Name the plasma proteins that bind thyroid hormones.

Albumin, transthyretin (formerly called thyroxine-binding prealbumin), and thyroxine-binding globulin (TBG).

Free T4 feeds back to inhibit the secretion of what hormone?

TSH

How is T4 metabolized in the periphery?

T4 is deiodinated in the liver and other tissues to provide a local supply of T3 is what happens to T4.

Which three deiodinases act on thyroid hormones?

D1, D2, and D3

What inhibits deiodinases?

Various drugs, selenium deficiency, and nonthyroidal illnesses suppress deiodinases.

How is thyroid function primarily regulated?

By variations in the circulating level of pituitary TSH.

What stimulates TSH secretion?

TRH – Thyrotropin Releasing Horomone

What inhibits TSH secretion?

Circulating free T4 and T3 inhibit TSH secretion

What does TSH activate?

Adenylyl cyclase through Gs and phospholipase C (PLC).

What is a goiter?

Enlargement of the thyroid that always becomes detectably enlarged due to prolonged TSH stimulation.

Name other factors, in addition to TSH, that affect Thyroid Growth.

insulin-like growth factor I (IGF-I) and epidermal growth factor (EGF) promote growth, whereas interferon-γ and tumor necrosis factor-α inhibit growth

What are some effects of thyroid hormones?

Stimulation of O2 consumption, affect growth and development, help regulate lipid metabolism, and increase absorption of carbohydrates from the intestine.

What is the mechanism of action for thyroid hormones?

T3 binds to thyroid hormone receptors (TRs) then migrates to the nucleus and binds to DNA, increasing or decreasing the expression of a variety of different genes.

What is the calorigenic action of thyroid hormones?

T4 and T3 increase O2 consumption by almost all metabolically active tissues.

What reactions occur secondary to calorigenesis?

Nitrogen excretion is increased, food intake is not increased, endogenous protein and fat stores are catabolized and weight is lost.

How do thyroid hormones effect the cardiovascular system?

T3 is not formed from T4 in cardiac myocytes, but circulating T3 enters myocytes, combines with its receptors, and enters the nucleus, where the complex promotes the expression of some genes and inhibits the expression of others, resulting in an increased heart rate and force of contraction.

How is the nervous system affected by hypothroidism?

Hypothyroidism causes the cerebrspinal fluid (CSF) protein level to be elevated and mentation to be slow. These can be reversed with thyroid hormones

Relate thyroid hormones to catecholamines

The actions of thyroid hormones and the catecholamines norepinephrine and epinephrine are intimately interrelated. The toxicity of the catecholamines is markedly increased in rats treated with T4. The cardiovascular effects, tremulousness, and sweating that are seen in the setting of excess thyroid hormones can be reduced or abolished by sympathectomy.

How does reduced thyroid function present?

Reduced thyroid function presents in a number of ways: hair is coarse and sparse, the skin is dry and yellowish (carotenemia), and cold is poorly tolerated. Mentation is slow, memory is poor.

What causes hyperthyroidism?

Hyperthyroidism is most commonly caused by Graves disease (Graves hyperthyroidism). This is an autoimmune disease, more common in women, in which antibodies to the TSH receptor stimulate the receptor.