Thyroid Anatomy and Pathology Quiz

Thyroid Gland Introduction

• Thyroid gland extends from the oblique line of thyroid cartilage above to the 6th tracheal ring below
• Thyroid isthmus overlies tracheal rings 2 to 4
• The Zuckerkandl nodule is a lateral hump in the middle of the thyroid lobe
  • It hides the recurrent laryngeal nerve (RLN), increasing the difficulty of its dissection
• The superior thyroid artery originates from the external carotid artery (ECA)
  • It accompanies the external laryngeal nerve
• The inferior thyroid artery (ITA) emerges from the thyrocervical trunk, a branch of the first part of the subclavian artery
  • ITA is the primary blood supply to the thyroid gland
  • It is closely related to the RLN
• The thyroidea ima artery, present in 3% of people, arises from the aortic arch
  • It enters the thyroid through the lower border of the isthmus
• The superior thyroid vein drains into the internal jugular vein (IJV)
• The inferior thyroid vein and the esophageal and tracheal branches drain into the left innominate vein
• Lymphatic drainage of the thyroid proceeds through lymph nodes in regions VI and VII, then III, IV, V, II, and lastly I

Three Deep Fasciae

• The pre-tracheal fascia directly surrounds the thyroid gland
  • It causes the thyroid to move with deglutition
  • It facilitates retro-sternal extension into the superior mediastinum
• Berry's ligament, a tough, postero-medial part of the pre-tracheal fascia, anchors the thyroid lobe to the trachea
  • It hides the RLN just before it enters the larynx
  • It contains a small artery and vein, requiring careful dissection during surgery

Thyroid Nerves

• The external laryngeal nerve, a sub-branch of the superior laryngeal nerve of the vagus nerve, resides in Joll's triangle
  • It supplies the cricothyroid muscle, the only intrinsic laryngeal muscle outside the larynx
  • Injury during ligation of the superior thyroid vessels can lead to paralysis of the ipsilateral cricothyroid
    • This results in vocal fatigue and inability to produce high-pitched sounds
• The internal laryngeal nerve, also a branch of the superior laryngeal nerve, is purely sensory, supplying the larynx above the vocal folds
• The recurrent (inferior) laryngeal nerve (RLN), a branch of the vagus nerve, is a mixed nerve
  • It supplies motor innervation to all intrinsic laryngeal muscles except the cricothyroid
  • It innervates the sensory aspect of the larynx below the vocal folds
  • Injury to the RLN can lead to different degrees of respiratory distress and voice changes, depending on the extent of the damage

Imaging: Investigations for Thyroid Abnormalities

• Plain X-ray of the neck and upper chest reveals soft tissue shadow, potentially indicating a retrosternal goiter
• Ultrasound (US) of the neck is the most critical investigation
  • It differentiates cystic from solid nodules and detects clinically non-palpable nodules
• Computed tomography (CT) of the neck provides information on retrosternal goiters and cervical lymph nodes

Other Thyroid Investigations

• Biopsy, either fine needle aspiration (FNAC) or Tru-cut needle biopsy, is performed for suspected malignancy
• Indirect laryngoscopy assesses vocal cord function
• TSH, FT3, and FT4 are routine investigations for thyroid function
• Thyroid auto-antibodies are assessed to rule out Hashimoto's thyroiditis
• Thyroid tumor markers, particularly calcitonin for medullary cancer, are utilized in the diagnostic process

Clinical Approach to Goiter

• Goiter is defined as an enlarged thyroid gland, regardless of the underlying cause or function
• A thyroid swelling is typically located in the lower anterior neck, deep to the sternomastoid muscle
• Goiters typically have a butterfly shape, although enlargement can be unilateral or asymmetrical
• A goiter moves up and down with deglutition, a characteristic differentiating it from other neck masses

Clinical Types of Goiters

• Simple Goiter:
  • Diffuse:
    • Physiological:
      • May be transient in adolescents, pregnant women, and those with iodine deficiency
    • Colloidal:
      • Characterized by an increase in colloid within the follicles of thyroid gland, related to iodine deficiency
  • Multinodular:
    • Presence of multiple nodules within the thyroid gland
  • Solitary Nodule:
    • Presence of a single nodule within the thyroid gland
  • Recurrent Nodular:
    • Occurs when nodules reappear after previous treatment
• Toxic Goiter:
  • Also known as hyperthyroidism
  • Diffuse (1):
    • Graves' Disease:
      • The most common form of hyperthyroidism, caused by an autoimmune disorder
  • Multinodular (2):
    • Marine Lenhart Syndrome:
      • Hyperthyroidism due to multiple nodules within the thyroid gland
  • Solitary Nodule:
    • Plummer's Disease:
      • Hyperthyroidism caused by a single hyperactive nodule
  • Recurrent Nodular:
    • Hyperthyroidism related to the reappearance of nodules after prior treatment
• Special Goiter:
  • Thyroiditis:
    • Inflammation of the thyroid gland
  • Neoplastic:
    • Benign:
      • Adenoma:
        • A non-cancerous tumor of the thyroid
    • Malignant:
      • Cancerous tumors of the thyroid
  • Autoimmune:
    • Hashimoto's thyroiditis, an autoimmune disease causing inflammation and eventual destruction of the thyroid gland
  • Congenital (Dyshormonogenesis):
    • Disorders that impair the production of thyroid hormones

Goiter: Symptom Evaluation

• Sudden Appearance or Rapid Increase in Size:
  • May indicate malignancy, Graves' disease, or thyroiditis
• Symptoms Due to Thyroid Swelling:
  • Vocal Changes:
    • Hoarseness, dysphonia, stridor, or aphonia due to compression of the recurrent laryngeal nerve
  • Difficulty Swallowing (Dysphagia):
    • Caused by pressure on the esophagus
  • Respiratory Distress:
    • Stridor, wheezing, or shortness of breath (dyspnea) due to compression the trachea
    • Postural dyspnea, stridor, cough, wheezing, choking, and dysphagia may indicate retro-sternal extension
  • Neck Pain and Tenderness:
    • Could suggest thyroiditis or tumor involvement
  • Visual Disturbances:
    • Exophthalmos (protruding eyeballs), chemosis (swelling of the conjunctiva), ophthalmoplegia (eye muscle paralysis), and dilated and congested conjunctival blood vessels
  • Tremor:
    • Fine tremor of the eyelids and hands may indicate hyperthyroidism
• The Hands:
  • Moist and warm in hyperthyroidism, fine tremors indicate hyperthyroidism
• Lower Limbs:
  • Pretibial myxedema (non-pitting edema with deep purple chins) is suggestive of hyperthyroidism
  • Berry's Sign:
    • Displacement of the carotid artery backwards and outwards by the goiter

Retro-sternal Extension of Goiter: Clinical Findings

- History:
    - Postural dyspnea, stridor, cough, wheezing, choking, dysphagia, etc.
- Physical Examination:
    - Dilated veins in front of the neck and sternum
    - Enlarged thyroid with non-visible lower border on swallowing
    - Impalpable lower border on palpation
    - Dullness on percussion over the manubrium sterni
    - Flushing of the skin and dilatation of the external jugular vein when raising the arms or hyperextending the neck (positive Pemberton's sign) 
- Investigations:
    - Plain X-ray
    - CT scan
    - Thyroid scan

Differentiating Primary from Secondary Thyroid Hyperthyroidism

Goiter: Clinical Criteria

Multinodular Goiter: Complications

• Toxicity
• Malignancy
• Retrosternal extension
• Pressure
• Cyst formation
• Calcification
• Hemorrhage
• Infection (rare due to extensive vascularity)

Goiter: Criteria for Malignant Transformation

Thyroid Tumors

Benign Tumors of the Thyroid

Pathological Classification of Benign Thyroid Tumors

Malignant Tumors of the Thyroid

Suspicion of Thyroid Malignancy

• Enlarged, painless thyroid lesion with one or more of the following:
  • Radiation exposure
  • Male gender, older age, or younger age
  • Rapid increase in size
  • Previous thyroid cancer
  • Lymphadenopathy
  • Evidence of local invasion (vocal cord paralysis, dysphagia, or firm, fixed nodules)

Familial Syndromes Increasing Thyroid Cancer Risk

• Familial non-medullary thyroid cancer:

Thyroid Cancer: Incidence by Type

Thyroid Cancer: Classification

• Differentiated tumors of follicular origin (90-95%): Papillary carcinoma, follicular carcinoma, Hürthle cell carcinoma
• From parafollicular cells (2-10%): Medullary thyroid carcinoma (MTC)
• Poorly differentiated (5-15%): Anaplastic thyroid carcinoma (ATC)

Oncogenes Associated with Thyroid Carcinoma

1. RET oncogene: Papillary (PTC) & MTC.
2. Mutated RAS oncogene: Follicular thyroid carcinoma (FTC)
3. Mutated p53 gene: Anaplastic thyroid carcinoma (ATC).

Papillary Thyroid Carcinoma (PTC)

• The most common histological type of thyroid malignancy (80%)
• Predominant thyroid cancer in children
• May be linked to radiation exposure of the neck
• Origin: Oxyphilic cells of the thyroid gland
• Spreads via lymphatics
• Diagnosis: FNAC (20% malignant)
• Often multifocal and bilateral
• Treatment:
  • Total thyroidectomy + Modified radical neck dissection (if with palpable cervical LNs)

Medullary Thyroid Carcinoma (MTC)

• Peak incidence at 50-60 years
• Origin: Parafollicular or C cells of the thyroid (neuroectodermal)
• Secretes calcitonin (95%) and carcinoembryonic antigen (CEA) (85%)
• Sporadic (90%): Unifocal, usually at 45 years, worse prognosis
• Familial (10%): Multifocal, usually at 35 years, better prognosis, associated with:
  • MEN IIA (Sipple's syndrome):
    • MTC, hyperplastic parathyroid, and pheochromocytoma
  • MEN IIB:
    • MTC, pheochromocytoma, ganglioneuromatosis, and Marfan's syndrome
• Spread:
  • Lymphatics (neck and superior mediastinum)
  • Blood → liver, bone (osteoplastic), and lung
  • Local invasion
• Diagnosis:
  • Serum calcitonin and CEA levels
  • FNAC: Characteristic amyloid stroma
• Treatment:
  • Total thyroidectomy
  • Modified radical neck dissection (for palpable cervical lymph nodes or tumor size >2cm)
  • Screening for pheochromocytoma (MEN II) and resection before thyroid surgery
• Follow-up:
  • Serum calcitonin and CEA levels
• Prognosis:
  • Best → worst:
    • Familial non-MEN MTC → MEN IIA → sporadic cases → MEN IIB

Anaplastic Thyroid Carcinoma (ATC)

• Uncommon, affecting older patients
• May arise from a previously well-differentiated thyroid carcinoma
• Often presents as a rapidly growing, painless goiter in patients with a history of a long-standing goiter
• Treatment:
  • Tracheostomy and total thyroidectomy are difficult due to the risk of injury to neck structures
  • External radiation may temporarily control local effects
  • Limited effect of systemic chemotherapy (Adriamycin); no hormonal therapy
• Extremely poor prognosis with a mean life expectancy of 6-9 months.
• Death usually results from local invasion of vital cervical structures and airway compression.

Thyroid Lymphoma

• Non-Hodgkin B-cell lymphoma
• Hashimoto's thyroiditis increases risk
• Treatment:
  • Chemotherapy and radiotherapy
  • Surgical intervention may be required in some cases

Test your knowledge on the anatomy and pathology related to the thyroid gland. This quiz covers topics such as the external laryngeal nerve, goiter types, and diagnostic imaging techniques. Perfect for medical students or health professionals preparing for exams.