Thyroid Pathology and Pharmacology
Comprehensive Study Guide and Clinical Self-Assessment
1. Anatomy and Clinical Assessment
The thyroid gland is located in the anterior region of the neck, caudal to the thyroid cartilage and ventral
to the trachea. It is a frequent reason for consultation in primary care due to its high prevalence,
especially in women. Symptoms are usually nonspecific, so diagnosis requires high clinical suspicion and
a thorough physical examination.
Anatomical Characteristics
• Weight: 15 - 20 grams.
• Morphology: Right lobe, left lobe, and isthmus. It may present an inconstant cranial extension
(Pyramidal lobe or Lalouette/Delphian lobe).
• Average lobe size: 3-4 cm (Length) x 1.5-2 cm (Transverse) x 1-2 cm (Anteroposterior). Mnemonic
rule: 3 x 2 x 1 cm.
Vascularization and Drainage
• Blood flow: 5 ml/Kg/minute (Approx. 100 ml/minute).
• Arterial supply: Superior thyroid arteries (branches of the external carotid artery) and inferior thyroid
arteries (branches of the thyrocervical trunk of the subclavian). Inconstantly, the thyroid ima artery (of
Neubauer) arises from the aortic arch.
• Venous drainage: Superior and middle thyroid veins (drain into the internal jugular) and inferior
thyroid veins (drain into the brachiocephalic vein).
• Lymphatic drainage: Abundant network, mainly towards pretracheal, paratracheal, and prelaryngeal
nodes.
, Cervical Physical Examination
The patient should be relaxed, seated, with the head slightly flexed. The examiner stands behind,
surrounding the neck with their hands to delimit the lobes with the fingertips. The patient should be
asked to swallow to assess the mobility of the gland over deep planes.
Goiter Classification (WHO):
• 0A: No goiter.
• 0B: Goiter not visible with the neck in hyperextension; only palpable.
• I: Goiter palpable and visible only with the neck in hyperextension.
• II: Goiter visible with the neck in normal position.
• III: Large goiter, visible from a distance.
2. Physiology and Regulation
Thyroid function is finely regulated by the Hypothalamic-Pituitary-Thyroid Axis. The hypothalamus
secretes TRH (tripeptide), which stimulates the anterior pituitary to secrete TSH (glycopeptide). TSH
stimulates the thyroid gland to synthesize hormones T3 and T4, which exert negative feedback on the
pituitary and hypothalamus.
Biosynthesis of Thyroid Hormones
1. Uptake of plasma iodide.
2. Oxidation of iodide by the enzyme peroxidase and H₂O₂.
3. Organification (iodination of tyrosine residues of thyroglobulin to form MIT and DIT).
4. Coupling (MIT + DIT = T3; DIT + DIT = T4).
5. Endocytosis, proteolysis of thyroglobulin in phagolysosomes, and release of T3 and T4.
Production and Circulation
• The thyroid gland produces 100% of T4 and only 25% of T3.
• 75% of T3 (biologically active hormone) is produced in peripheral tissues through the deiodination of
T4.
• In plasma, they circulate mainly bound to proteins: TBG (Thyroxine-binding globulin - 80%),
Prealbumin (20%), and Albumin (10%).
Comprehensive Study Guide and Clinical Self-Assessment
1. Anatomy and Clinical Assessment
The thyroid gland is located in the anterior region of the neck, caudal to the thyroid cartilage and ventral
to the trachea. It is a frequent reason for consultation in primary care due to its high prevalence,
especially in women. Symptoms are usually nonspecific, so diagnosis requires high clinical suspicion and
a thorough physical examination.
Anatomical Characteristics
• Weight: 15 - 20 grams.
• Morphology: Right lobe, left lobe, and isthmus. It may present an inconstant cranial extension
(Pyramidal lobe or Lalouette/Delphian lobe).
• Average lobe size: 3-4 cm (Length) x 1.5-2 cm (Transverse) x 1-2 cm (Anteroposterior). Mnemonic
rule: 3 x 2 x 1 cm.
Vascularization and Drainage
• Blood flow: 5 ml/Kg/minute (Approx. 100 ml/minute).
• Arterial supply: Superior thyroid arteries (branches of the external carotid artery) and inferior thyroid
arteries (branches of the thyrocervical trunk of the subclavian). Inconstantly, the thyroid ima artery (of
Neubauer) arises from the aortic arch.
• Venous drainage: Superior and middle thyroid veins (drain into the internal jugular) and inferior
thyroid veins (drain into the brachiocephalic vein).
• Lymphatic drainage: Abundant network, mainly towards pretracheal, paratracheal, and prelaryngeal
nodes.
, Cervical Physical Examination
The patient should be relaxed, seated, with the head slightly flexed. The examiner stands behind,
surrounding the neck with their hands to delimit the lobes with the fingertips. The patient should be
asked to swallow to assess the mobility of the gland over deep planes.
Goiter Classification (WHO):
• 0A: No goiter.
• 0B: Goiter not visible with the neck in hyperextension; only palpable.
• I: Goiter palpable and visible only with the neck in hyperextension.
• II: Goiter visible with the neck in normal position.
• III: Large goiter, visible from a distance.
2. Physiology and Regulation
Thyroid function is finely regulated by the Hypothalamic-Pituitary-Thyroid Axis. The hypothalamus
secretes TRH (tripeptide), which stimulates the anterior pituitary to secrete TSH (glycopeptide). TSH
stimulates the thyroid gland to synthesize hormones T3 and T4, which exert negative feedback on the
pituitary and hypothalamus.
Biosynthesis of Thyroid Hormones
1. Uptake of plasma iodide.
2. Oxidation of iodide by the enzyme peroxidase and H₂O₂.
3. Organification (iodination of tyrosine residues of thyroglobulin to form MIT and DIT).
4. Coupling (MIT + DIT = T3; DIT + DIT = T4).
5. Endocytosis, proteolysis of thyroglobulin in phagolysosomes, and release of T3 and T4.
Production and Circulation
• The thyroid gland produces 100% of T4 and only 25% of T3.
• 75% of T3 (biologically active hormone) is produced in peripheral tissues through the deiodination of
T4.
• In plasma, they circulate mainly bound to proteins: TBG (Thyroxine-binding globulin - 80%),
Prealbumin (20%), and Albumin (10%).
