From the 1st and 2nd pharyngeal arches (not from pouches)
Thyrotropin-releasing factor (TRF) – released from the hypothalamus; acts on the anterior pituitary gland and causes release of TSH
Thyroid-stimulating hormone (TSH) – released from the anterior pituitary gland; acts on the thyroid gland to release T3 and T4 (through a mechanism that involves ↑ cAMP)
TRF and TSH release are controlled by T3 and T4 through a negative feedback loop
Superior thyroid artery – 1st branch off external carotid artery
Inferior thyroid artery – off thyrocervical trunk; supplies both the inferior and superior parathyroids
• Ligate close to thyroid to avoid injury to parathyroid glands with thyroidectomy
Ima artery – occurs in 1%, arises from the innominate or aorta and goes to the isthmus
Superior and middle thyroid veins – drain into internal jugular vein
Inferior thyroid vein – drains into innominate vein
Superior laryngeal nerve
• Motor to cricothyroid muscle
• Runs lateral to thyroid lobes
• Tracks close to superior thyroid artery but is variable
Injury results in loss of projection and easy voice fatigability (opera singers)
Recurrent laryngeal nerves (RLNs)
• Motor to all of larynx except cricothyroid muscle
Run posterior to thyroid lobes in the tracheoesophageal groove
• Can track with inferior thyroid artery but are variable
• Left RLN loops around aorta; right RLN loops around innominate artery
• Injury results in hoarseness; bilateral injury can obstruct airway → need emergency tracheostomy
• Non-recurrent laryngeal nerve – in 2%; more common on the right
• Risk of injury is higher for a non-recurrent laryngeal nerve during thyroid surgery
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Ligament of Berry – posterior medial suspensory ligament close to RLNs; need careful dissection
Thyroglobulin – stores T3 and T4 in colloid
• Plasma T4:T3 ratio is 15:1; T3 is the more active form (is tyrosine + iodine)
• Most T3 is produced in periphery from T4 to T3 conversion by deiodinases
Peroxidases link iodine and tyrosine together
Deiodinases separate iodine from tyrosine
Thyroxine-binding globulin – thyroid hormone transport; binds the majority of T3 and T4 in circulation
TSH – most sensitive indicator of gland function
Tubercles of Zuckerkandl – most lateral, posterior extension of thyroid tissue
• Rotate medially to find RLNs
• This portion is left behind with subtotal thyroidectomy because of proximity to RLNs
Parafollicular C cells – produce calcitonin
Thyroxine treatment – TSH levels should fall 50%; osteoporosis long-term side effect
Post-thyroidectomy stridor – open neck and remove hematoma emergently → can result in airway compromise; can also be due to bilateral RLN injury → would need emergent tracheostomy
THYROID STORM
Symptoms: ↑ HR, fever, numbness, irritability, vomiting, diarrhea, high-output cardiac failure (most common cause of death)
Most common after surgery in patient with undiagnosed Graves’ disease
Can be precipitated by anxiety, excessive gland palpation, adrenergic stimulants
Tx: β-blockers (first line), PTU, Lugol’s solution (KI), cooling blankets, oxygen, glucose
• Emergent thyroidectomy rarely indicated
Wolff–Chaikoff effect – very effective for thyroid storm; patient given high doses of iodine (Lugol’s solution, potassium iodide), which inhibits TSH action on thyroid and inhibits organic coupling of iodide, resulting in less T3 and T4 release
ASYMPTOMATIC THYROID NODULE
90% of thyroid nodules are benign; female predominance
Get FNA (best initial test) and thyroid function tests
• Determinant in 80% → follow appropriate treatment
• Shows follicular cells → lobectomy (10% CA risk; see Thyroid CA section)
• Shows thyroid CA → thyroidectomy or lobectomy and appropriate treatment (see Thyroid CA section)
• Shows cyst fluid → drain fluid
• If it recurs or is bloody → lobectomy
• Shows colloid tissue → most likely colloid goiter; low chance of malignancy (< 1%)
• Tx: thyroxine; lobectomy if it enlarges
• Shows normal thyroid tissue and TFTs are elevated → likely solitary toxic nodule
• Tx: if asymptomatic can just monitor; PTU and 131I if symptomatic
• Indeterminant in 20% → get radionuclide study
• Hot nodule → Tx: if asymptomatic can monitor; PTU and 131I if symptomatic
• Cold nodule → lobectomy (more likely malignant than hot nodule)
Goiter
• Any abnormal enlargement
• Most identifiable cause is iodine deficiency; Tx: iodine replacement
• Diffuse enlargement without evidence of functional abnormality = nontoxic colloid goiter
• Unusual to have to operate unless goiter is causing airway compression or there is a suspicious nodule
• Tx: subtotal or total thyroidectomy for symptoms or if suspicious nodule; subtotal has decreased risk of RLN injury
Substernal goiter
• Usually secondary (vessels originate from superior and inferior thyroid arteries)
• Primary substernal goiter – rare (vessels originate from innominate artery)
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