thyroid gland overview

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Thyroid Gland - Overview

1. Anatomy

The thyroid is a butterfly-shaped endocrine gland located in the anterior neck, inferior to the larynx and anterior to the trachea. It overlies cervical levels C5-T1.

Gross structure:

Two lateral lobes connected by a narrow isthmus, which crosses anterior to the 2nd and 3rd tracheal cartilages
The lobes cover the anterolateral surfaces of the trachea, the cricoid cartilage, and the lower thyroid cartilage
A pyramidal lobe may extend superiorly from the isthmus in ~50% of individuals, representing a remnant of the thyroglossal duct
Lies deep to the strap muscles (sternohyoid, sternothyroid, and omohyoid) within the visceral (pretracheal) compartment

Thyroid gland - anterior and transverse views with ultrasound and nuclear medicine scan

Fig. 8.181 - Thyroid Gland in the Anterior Triangle of Neck showing (A) Anterior view, (B) Transverse section, (C) Ultrasound compound axial, (D) Ultrasound axial with carotid artery, (E) Nuclear medicine pertechnetate scan. - Gray's Anatomy for Students

2. Embryology

The thyroid develops from an endodermal evagination of the pharyngeal floor (foramen cecum at the base of the tongue) and descends along the thyroglossal duct to its final position in the anterior neck. The thyroglossal duct normally obliterates; remnants can persist as:

A thyroglossal duct cyst (midline neck mass)
A lingual thyroid (ectopic functional thyroid at the base of tongue)
A fistula connecting to the foramen cecum

Parafollicular C cells originate from the neural crest (via the ultimobranchial body), not from endoderm.

Gray's Anatomy for Students, p. 1157
Robbins, Cotran & Kumar Pathologic Basis of Disease, p. 994

3. Vasculature

Thyroid gland vasculature showing inferior thyroid artery and inferior thyroid veins relative to the cricoid cartilage

Fig. 20-18 - Thyroid vascular anatomy. - Junqueira's Basic Histology

Vessel	Origin	Notes
Superior thyroid artery	First branch of external carotid artery	Descends along the thyrohyoid muscle to the superior pole; anterior branch anastomoses across the isthmus with its contralateral twin
Inferior thyroid artery	Thyrocervical trunk (from subclavian artery)	Passes posterior to the carotid sheath; closely related to the recurrent laryngeal nerve - surgically important
Superior thyroid vein	Drains upper pole	Tributary of internal jugular vein
Middle thyroid vein	Drains lateral gland	Tributary of internal jugular vein
Inferior thyroid veins	Drain lower pole	Drain into left brachiocephalic vein
Thyroidea ima artery	Aortic arch or brachiocephalic (variable)	Present in ~10%; can bleed during tracheotomy

Important surgical relationship: The recurrent laryngeal nerve (RLN) runs in the groove between the trachea and esophagus and is at risk during inferior thyroid artery ligation in thyroidectomy.

Gray's Anatomy for Students, p. 1158

4. Histology

The thyroid is covered by a fibrous capsule that sends septa inward, dividing it into lobules and carrying blood vessels, nerves, and lymphatics.

Follicular units: Each lobule contains 20-40 colloid-filled follicles. Key features:

Feature	Detail
Follicular cell (thyrocyte) shape	Squamous (inactive) to low columnar (active) - controlled by TSH
Colloid	Gelatinous, acidophilic; contains thyroglobulin (660 kDa glycoprotein) - the hormone precursor
Hormone storage	The thyroid is unique among endocrine glands: large quantities of secretory product are stored extracellularly in follicle lumens (up to 3 months' supply)
Stroma	Sparse reticular connective tissue with fenestrated capillaries for rapid hormone transfer

Two cell populations:

Follicular cells (thyrocytes) - synthesize T3 and T4
Parafollicular cells (C cells) - scattered between follicles or within the follicular epithelium; synthesize calcitonin

Thyroid histology H&E - follicles (colloid-filled) with fibrous capsule (C) and septa (S)

H&E section of thyroid parenchyma: C = capsule, S = fibrous septa; multiple round follicles filled with pink gelatinous colloid. - Junqueira's Basic Histology, p. 1034-1035

Junqueira's Basic Histology, p. 1034-1035

5. Hormone Synthesis and Regulation

Hypothalamus-Pituitary-Thyroid (HPT) Axis

HPT axis and mechanism of thyroid hormone action: TRH from hypothalamus → TSH from pituitary → thyroid produces T3/T4 → negative feedback. TSH acts via G-protein/cAMP. T3/T4 enter cells and act on nuclear THR/RXR to activate gene expression.

Fig. 24.8 - HPT axis and thyroid hormone mechanism of action. - Robbins, Cotran & Kumar, p. 995

Steps:

TRH (thyrotropin-releasing hormone) is secreted from the paraventricular nucleus of the hypothalamus
TRH stimulates anterior pituitary thyrotrophs to release TSH (thyroid-stimulating hormone)
TSH binds its receptor on follicular cells → activates Gs protein → ↑ cAMP → stimulates thyroid growth and hormone synthesis and release
Negative feedback: Rising T3/T4 suppress both TRH (hypothalamus) and TSH (pituitary)

Thyroid Hormone Synthesis Steps (within the follicular cell):

Iodide trapping - active Na+/I- symporter (NIS) concentrates iodide
Oxidation - thyroid peroxidase (TPO) oxidizes I- to I2 (reactive iodine)
Organification - I2 is incorporated onto tyrosine residues of thyroglobulin → monoiodotyrosine (MIT) and diiodotyrosine (DIT)
Coupling - DIT + DIT → T4; MIT + DIT → T3 (within thyroglobulin stored as colloid)
Secretion - Follicular cells endocytose colloid; lysosomes cleave T3/T4 from thyroglobulin; free hormones are secreted into capillaries

Circulating Hormones:

Most T3 and T4 travel bound to thyroxine-binding globulin (TBG), transthyretin, and albumin
Only free (unbound) hormone is biologically active
~80% of circulating T3 is produced by peripheral deiodination of T4 (primarily in liver and kidney)
T3 has ~10x greater receptor affinity than T4 and is the more potent hormone
Robbins, Cotran & Kumar, p. 994-996

6. Actions of Thyroid Hormones (T3/T4)

T3/T4 bind nuclear thyroid hormone receptors (THR-alpha and THR-beta), which heterodimerize with RXR and bind thyroid response elements (TREs) to regulate gene transcription. Major effects:

System	Effect
Metabolism	Increases basal metabolic rate; stimulates carbohydrate and lipid catabolism; promotes protein synthesis
Cardiovascular	Increases heart rate and cardiac output; potentiates catecholamine sensitivity
Nervous system	Required for normal brain development in fetus and neonate (critical window)
Skeletal	Promotes normal bone growth and maturation
Thermogenesis	Increases heat production via uncoupling of oxidative phosphorylation

7. Calcitonin (C cells)

Secreted by parafollicular (C) cells
Opposes PTH: promotes calcium deposition into bone; inhibits osteoclast-mediated bone resorption
Results: lowers serum calcium
Clinically: C cell hyperplasia and medullary thyroid carcinoma (MTC) arise from these cells; MTC is associated with MEN2A and MEN2B (RET proto-oncogene mutations)

8. Clinical Relevance - Common Disorders

Category	Examples
Hyperthyroidism / Thyrotoxicosis	Graves' disease (TSH-receptor autoantibodies), toxic multinodular goiter, toxic adenoma
Hypothyroidism	Hashimoto's thyroiditis (most common in iodine-sufficient regions), iodine deficiency (most common worldwide), iatrogenic (post-thyroidectomy/radioiodine)
Goiter	Enlargement from TSH stimulation (iodine deficiency, goitrogens like propylthiouracil and methimazole)
Thyroid nodules/neoplasms	Papillary carcinoma (most common), follicular carcinoma, medullary carcinoma, anaplastic carcinoma

Goitrogens - agents that suppress thyroid hormone synthesis, leading to compensatory TSH elevation and glandular enlargement:

Propylthiouracil (PTU) - inhibits TPO (oxidation) and peripheral T4→T3 conversion
Methimazole - inhibits TPO
Large doses of iodide (Wolff-Chaikoff effect) - blocks hormone release acutely

Sources:

Gray's Anatomy for Students (9780323934237)
Junqueira's Basic Histology - Text and Atlas, 17e (9781264930395)
Robbins, Cotran & Kumar - Pathologic Basis of Disease (9780443264528)

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