Nerve Supply of the Thyroid Gland
Overview
The thyroid gland receives only vasomotor (sympathetic) nerve fibres — there is no secretomotor innervation from the nervous system. Thyroid hormone secretion is regulated by TSH (thyroid-stimulating hormone) from the anterior pituitary, not by neural input.
Sympathetic Supply
Origin: Superior, middle, and inferior cervical sympathetic ganglia
Route: Sympathetic fibres travel along the superior and inferior thyroid arteries as perivascular plexuses to reach the gland
Function:
- Vasomotor — regulate blood flow to the gland
- May modulate some secretory activity indirectly through vascular control
Parasympathetic Supply
The vagus nerve contributes some fibres via the laryngeal nerves, but these are primarily for vasodilation rather than secretomotor function.
Nerves in Close Anatomical Relation (Not Supplying the Gland)
These nerves do not supply the thyroid but are critically at risk during thyroid surgery:
1. Recurrent Laryngeal Nerve (RLN)
- Motor to all intrinsic laryngeal muscles except cricothyroid
- Sensory to larynx below the vocal cords
- Runs in the tracheo-oesophageal groove; crosses the inferior thyroid artery posteromedially
- Enters the larynx beneath the lower border of the inferior constrictor at the level of the cricothyroid joint
- Injury → hoarseness (unilateral); respiratory distress/stridor (bilateral)
2. External Branch of the Superior Laryngeal Nerve (EBSLN)
- Motor to cricothyroid muscle only — tenses the vocal cords; responsible for high-pitched vocalisation
- Descends medial to the superior thyroid artery before the artery branches at the upper pole
- Injury → loss of high-pitched sounds; voice fatigue; subtle dysphonia (may go unnoticed until patient tries to sing)
- Particularly important in singers — Amelita Galli-Curci sign (famous soprano who lost high notes after thyroid surgery)
3. Internal Branch of the Superior Laryngeal Nerve
- Purely sensory — pierces the thyrohyoid membrane
- Not typically at risk during thyroid surgery
Summary
| Nerve | Function | Risk in Thyroid Surgery |
|---|---|---|
| Recurrent laryngeal nerve | Motor: all intrinsic laryngeal muscles except cricothyroid; Sensory: subglottic | HIGH — hoarseness or respiratory distress |
| External branch of SLN | Motor: cricothyroid only | MODERATE — loss of high-pitched voice |
| Sympathetic chain | Vasomotor to gland | LOW — Horner’s syndrome if damaged |