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Left lobe of thyroid gland

The left lobe of the thyroid gland is one of the two lateral lobes of the thyroid, a butterfly-shaped endocrine gland located in the anterior neck. It extends from the thyroid isthmus across the trachea and larynx upward toward the oblique line of the thyroid cartilage and downward to the level of the fifth or sixth tracheal ring. The left lobe is typically slightly larger than the right and is related laterally to the sternothyroid, sternohyoid, and omohyoid muscles, medially to the larynx, trachea, esophagus, and recurrent laryngeal nerve, and posteriorly to the carotid sheath structures (common carotid artery, internal jugular vein, vagus nerve). Its primary function is the production and secretion of thyroid hormones (T3, T4) and calcitonin, which regulate metabolism, growth, and calcium balance.

Synonyms

  • Lobus sinister glandulae thyroideae

  • Left thyroid lobe

Function

  • Produces and secretes thyroid hormones (T3, T4), regulating metabolism, growth, and development

  • Secretes calcitonin, involved in calcium homeostasis

  • Serves as part of the endocrine regulatory system, essential for energy balance and thermoregulation

Arterial Supply

  • Superior thyroid artery (branch of external carotid artery) – supplies upper pole

  • Inferior thyroid artery (branch of thyrocervical trunk from subclavian artery) – supplies lower pole

  • Occasional contribution from thyroid ima artery (when present)

Venous Drainage

  • Superior thyroid vein → internal jugular vein

  • Middle thyroid vein → internal jugular vein

  • Inferior thyroid vein → brachiocephalic veins

Nerve Supply

  • Derived from the cervical sympathetic trunk (vasomotor fibers)

  • Parasympathetic fibers from the vagus nerve (CN X) via the superior and recurrent laryngeal branches

  • Recurrent laryngeal nerve runs along the medial aspect of the lobe, important in surgery due to risk of injury

MRI Appearance

T1-weighted images:

  • Left thyroid lobe shows intermediate signal intensity relative to muscle

  • Surrounded by hyperintense fat in the neck spaces

  • Nodules or cysts may appear as focal alterations in signal

T2-weighted images:

  • Normal parenchyma is intermediate to slightly hyperintense compared to muscle

  • Cysts or colloid nodules are markedly hyperintense

  • Inflammatory or neoplastic lesions show variable signal intensity

STIR (Short Tau Inversion Recovery):

  • Fat suppression highlights edema, inflammation, or tumor infiltration

  • Normal thyroid remains intermediate signal; pathology (thyroiditis, carcinoma) appears hyperintense

T1 Post-Contrast (Gadolinium-enhanced):

  • Left lobe enhances homogeneously in normal glands

  • Nodules, thyroiditis, or malignancy may show heterogeneous or irregular enhancement

CT Appearance

Non-contrast CT:

  • Left thyroid lobe appears as a soft tissue density lateral to the trachea and anterior to the esophagus

  • Internal calcifications may be seen in nodules or malignancies

Contrast-enhanced CT (CECT):

  • Normal parenchyma enhances homogeneously

  • Nodules, tumors, or thyroiditis show heterogeneous enhancement

  • Useful for assessing size, retrosternal extension, tracheal/esophageal compression, or local invasion

MRI images

Left lobe of thyroid gland coronal MRI image -img-00000-00000