thyroid anatomy and physilogy

Question 1

Describe the anatomy of the thyroid gland.

Answer 1

The thyroid gland comprises a midline isthmus lying horizontally just below the cricoid cartilage and two lateral lobes that extend upward over the lower half of the thyroid cartilage. It is deep to the strap muscles of the neck and enclosed in the pretracheal fascia.

Question 2

What are pseudolobules in the thyroid gland?

Answer 2

Pseudolobules are fibrous septae that divide the thyroid gland into compartments.

Question 3

What are follicles in the thyroid gland?

Answer 3

Follicles are vesicles or acini in the thyroid gland that are surrounded by a capillary network.

Question 4

What lines the walls of the follicles in the thyroid gland?

Answer 4

The walls of the follicles in the thyroid gland are lined by cuboidal epithelium.

Question 5

What is the proteinaceous substance found in the lumen of the follicles in the thyroid gland?

Answer 5

The lumen of the follicles in the thyroid gland is filled with a proteinaceous colloid, which contains thyroglobulin (Tg).

Question 6

How does the thyroid gland develop?

Answer 6

The thyroid gland develops from the endoderm of the floor of the pharynx, with some contribution from the lateral pharyngeal pouches. The midline thyroid precursor gives rise to the thyroglossal duct.

Question 7

What are the parafollicular C cells in the thyroid gland?

Answer 7

The parafollicular C cells in the thyroid gland are derived from the ultimo-branchial body and give rise to calcitonin. They can undergo malignant transformation and cause medullary thyroid carcinoma.

Question 8

When does the fetal thyroid gland begin to concentrate and organify iodine?

Answer 8

The fetal thyroid gland begins to concentrate and organify iodine at about 10-12 weeks’ gestation.

Question 9

What is the role of maternal T4 in fetal thyroid function?

Answer 9

Maternal T4 crosses the placenta and provides the major thyroid hormone available to the fetus before the fetal thyroid gland becomes functional.

Question 10

What is the role of maternal T4 in fetal thyroid function?

Answer 10

Maternal T4 crosses the placenta and provides the major thyroid hormone available to the fetus before the fetal thyroid gland becomes functional.

Question 11

What is the role of maternal T4 in fetal thyroid function?

Answer 11

Maternal T4 crosses the placenta and provides the major thyroid hormone available to the fetus before the fetal thyroid gland becomes functional.

Question 12

What is the role of maternal T4 in fetal thyroid function?

Answer 12

Maternal T4 crosses the placenta and provides the major thyroid hormone available to the fetus before the fetal thyroid gland becomes functional.

Question 13

What should be assessed during a thyroid examination?

Answer 13

During a thyroid examination, one should inspect the neck for goitre, scars, asymmetry, or masses. Palpation should be done to assess size, tenderness, nodularity, and mobility of the thyroid gland. Lymph nodes in the neck should also be palpated.

Question 14

What is the role of iodine in the biosynthesis of thyroid hormones?

Answer 14

Iodine is actively transported into follicular thyrocytes and is organified onto tyrosyl residues in thyroglobulin (Tg) to produce monoiodotyrosine (MIT) and di-iodotyrosine (DIT).

Question 15

What is the role of thyroid peroxidase (TPO) in the synthesis of thyroid hormones?

Answer 15

Thyroid peroxidase (TPO) links two DITs to form the two-ringed structure T4, and MIT and DIT to form small amounts of T3 and reverse T3 (rT3).

Question 16

Where is T4 primarily synthesized?

Answer 16

T4 is primarily synthesized in the thyroid gland.

Question 17

Where is the majority of circulating T3 generated?

Answer 17

The majority of circulating T3 is generated in extraglandular tissues by the conversion of T4 to T3 by deiodinases.

Question 18

What inhibits the synthesis of thyroid hormones?

Answer 18

Various agents termed goitrogens, such as perchlorate, thiocyanate, thioureas, and large doses of iodine, can inhibit the synthesis of thyroid hormones.

Question 19

How are T4 and T3 transported in the blood?

Answer 19

T4 and T3 are almost entirely bound to plasma proteins in the blood. T4 is bound to thyroid-binding globulin (TBG), transthyretin (TTR), and albumin, while T3 is bound less avidly by TBG and not significantly by TTR.

Question 20

What is the significance of free hormone in thyroid function?

Answer 20

Only free or unbound hormone is available to tissues, and the metabolic state correlates more closely with the free hormone concentration in the plasma.

Question 21

What are some signs of thyroid disease that can be observed during a physical examination?

Answer 21

Signs of thyroid disease that can be observed during a physical examination include exophthalmos, proptosis, thyroid acropachy, pretibial myxoedema, hyperactivity, restlessness, and tremor in the hands.

Question 22

What is the role of iodine in the synthesis of thyroid hormones?

Answer 22

Iodine is actively transported into follicular thyrocytes and is organified onto tyrosyl residues in thyroglobulin (Tg) to produce monoiodotyrosine (MIT) and di-iodotyrosine (DIT).