Thyroid ABSITE Review 2026: Anatomy, Physiology, Pathology, and Malignancy

Thyroid Anatomy and Innervation

• 🩸 The superior thyroid artery, a branch of the external carotid, supplies the thyroid. The inferior thyroid artery branches from the thyrocervical trunk. An anomalous thyroid ima artery may arise directly from the innominate artery.
• 💧 The superior thyroid vein drains into the internal jugular vein, while the inferior thyroid vein drains into the innominate vein.
• 🗣️ The superior laryngeal nerve innervates the cricothyroid muscle, affecting vocal projection. Injury can lead to fatigue in speaking.
• 🫁 The recurrent laryngeal nerve, looping around the subclavian artery (right) or aorta (left), can cause vocal cord paralysis if injured. Bilateral injury can obstruct the airway.

Thyroid Physiology and Embryology

• 🧪 Thyroglobulin from follicular cells facilitates T3 and T4 synthesis. Calcitonin from C cells lowers serum calcium.
• ⚠️ Thyroglossal duct cysts, resulting from the pharyngeal lobe's extension, have malignant potential and should be resected.

Hyperthyroidism and Thyroiditis

• 📉 Hyperthyroidism is characterized by low TSH and elevated T3/T4. Medications include PTU (safe in first trimester pregnancy) and methimazole.
• ⚛️ Graves' disease is diagnosed via radioactive iodine uptake or TSH receptor antibodies. Management includes antithyroid medications, radioiodine ablation, or thyroidectomy.
• 🚨 Thyroid storm presents with hypothermia, CNS, and cardiovascular dysfunction. Treatment involves supportive care, cooling, and potentially radioiodine or thyroidectomy.
• 🦠 Common thyroiditis types include Hashimoto's (autoimmune, treated with hormone replacement), subacute granulomatous (painful, viral, managed with NSAIDs/steroids), and Riedel's (fibrotic, treated with hormone replacement/steroids).

Thyroid Nodules and Malignancy

• 🔍 Ultrasound evaluation for thyroid nodules focuses on echogenicity, calcifications, margins, vascularity, and T:W ratio. FNA is typically performed next.
• 📊 The Bethesda criteria guide FNA interpretation, with recommendations ranging from repeat FNA to lobectomy or total thyroidectomy.
• 🔬 Papillary thyroid cancer, the most common malignancy, spreads lymphatically and is diagnosed by FNA showing psammoma bodies and Orphan Annie nuclei. Staging is age-based.
• 🩸 Follicular thyroid cancer spreads hematogenously; FNA is unreliable, often requiring lobectomy for diagnosis. Management includes total thyroidectomy and potentially radioiodine therapy.
• 🧬 Medullary thyroid cancer originates from C cells, often associated with RET proto-oncogene mutations (MEN2A/2B). Management involves total thyroidectomy with neck dissection, monitored by CEA and calcitonin levels.
• 👶 Prophylactic thyroidectomy for MEN2 syndromes is recommended by age 5 for low/medium risk and within the first year for highest risk, with potential delays based on monitoring.

Quick Hits

• 🛡️ Graves' disease is associated with anti-TSH receptor antibodies; Hashimoto's with anti-TPO antibodies.
• 🚫 Medullary thyroid cancer does not respond to radioactive iodine.
• 📍 The recurrent laryngeal nerve is most commonly injured near the ligament of Treitz.
• 📍 The superior laryngeal nerve is injured near the superior pole of the thyroid during ligation of the superior thyroid artery.
• 🚽 Diarrhea is the most common symptom of elevated calcitonin.
• 📌 Delphian nodes (Level VI) are often the first site of metastasis in thyroid cancer.