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Anatomy of the thyroid, parathyroid and suprarenal (adrenal) glands
ENDOCRINE
jugular veins course over the isthmus. The strap muscles stretch and adhere to the thyroid when it enlarges so that, at operation, they may resemble thin layers of fascia. The larynx and trachea lie on the deep aspect of the thyroid, with the pharynx and oesophagus behind and the carotid sheath on either side. Two nerves are found in close relationship to the gland; the recurrent laryngeal nerve lies in the groove between the trachea and oesophagus, and the external branch of the superior laryngeal nerve lies deep to the upper pole and passes to the cricothyroid muscle.
Anatomy of the thyroid, parathyroid and suprarenal (adrenal) glands Harold Ellis
Parathyroid glands Thyroid gland
The parathyroid glands usually lie on the posterior aspect of the thyroid lobes (Figure 2). There are usually 4 parathyroid glands —a superior and inferior on either side—though the numbers vary from 2 to 6. Ninety percent are in close relationship to the thyroid, while 10%, almost invariably the inferior glands, are aberrant. Parathyroid glands are a yellowish-brown colour and each one is about the size of a split pea. The superior parathyroid is more constant in position than the inferior gland. It usually lies at the middle of the posterior border of the lobe of the thyroid, above the level at which the inferior thyroid artery crosses the recurrent laryngeal nerve. The inferior parathyroid is usually situated below the inferior artery near the lower pole of the thyroid gland. The next most common site is within 1 cm of the lower pole of the thyroid gland. Aberrant inferior parathyroids may descend along the inferior thyroid veins in front of the trachea, and may even track into the superior mediastinum in company with thymic tissues, for which there is an embryological explanation. The superior gland
The thyroid gland comprises three distinct regions (Figure 1): • the isthmus, which overlies the second and third rings of the trachea • the lateral lobes, each of which extends from the side of the thyroid cartilage downwards to the sixth tracheal ring • an inconstant pyramidal lobe, which projects upwards from the isthmus, usually on the left side, and represents a remnant of the embryological descent of the thyroid. The gland is enclosed in the pretracheal fascia, covered by the strap muscles and overlapped by the sternocleidomastoids. The anterior
Harold Ellis is Emeritus Professor of Surgery, London University, London, UK. He is Clinical Anatomist in the Division of Anatomy at King’s College London (Guy’s campus), London, UK.
The thyroid gland and its blood supply
Transverse section of the neck through C6 External carotid artery
Investing fascia
Superior thyroid artery and vein
Pretracheal fascia Anterior jugular vein
Internal jugular vein Sternocleidomastoid Sternohyoid Sternothyroid
Middle thyroid vein Inferior thyroid artery
Omohyoid External jugular vein
Thyrocervical trunk Subclavian artery
Inferior thyroid vein C6
Left brachiocephalic (innominate) vein
Carotid sheath (containing common carotid artery, internal jugular vein, and vagus nerve) with sympathetic chain behind
Pre-vertebral fascia
1
SURGERY
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© 2003 The Medicine Publishing Company Ltd
ENDOCRINE
Posterior aspect of the thyroid gland
Descent of the thyroid, showing possible sites of ectopic thyroid tissue or thyroglossal cysts Inferior constrictor muscle
Lingual thyroid Suprahyoid thyroglossal cyst
Thyroid gland Superior and inferior parathyroid glands Inferior thyroid artery
Track of thyroid descent and of a thyroglossal fistula
Recurrent laryngeal nerve
Thyroglossal cyst or ectopic thyroid
Oesophagus Trachea Pyramidal lobe 2 Retrosternal goitre
derives from the fourth branchial arch. The inferior gland develops from the third arch in company with the thymus. As the thymus descends, the inferior parathyroid is dragged down with it.
3
Blood supply The thyroid gland is supplied by three arteries and drained by three veins: • the superior thyroid artery arises from the external carotid and passes to the upper pole • the inferior thyroid artery arises from the thyrocervical trunk of the first part of the subclavian artery and passes behind the carotid sheath to the posterior aspect of the gland • the thyroidea ima artery is small and inconstant; when present, it arises from the aortic arch or the brachiocephalic artery • the superior thyroid vein drains the upper pole to the internal jugular vein • the middle thyroid vein drains from the lateral side of the gland to the internal jugular • the inferior thyroid veins (there are often several) drain the lower pole to the brachiocephalic veins. Besides these named branches, numerous small vessels pass to the thyroid from the pharynx and trachea so that, even when all the principal vessels are tied, the gland continues to bleed when cut across during a partial thyroidectomy.
a swelling at the tongue base, and the more common occurrence of a thyroglossal cyst or sinus along the pathway of descent. Such a sinus can be dissected from the midline of the neck along the front of the hyoid then backwards through the muscles of the tongue to the foramen caecum. (It is in such intimate contact with it that the centre of the hyoid must be excised during the dissection.) The thyroid may descend beyond its normal position in the neck down into the superior mediastinum (retrosternal goitre), the most common cause of a superior mediastinal mass.
Suprarenal (adrenal) glands The suprarenal glands cap the upper poles of the kidneys and lie against the corresponding crus of the diaphragm. The typical appearance on CT scan is that of an ‘arrow head’, lying against the easily identified crus on each side. The left is related anteriorly to the stomach across the lesser sac, whereas the right lies behind the right lobe of the liver and tucks medially behind the inferior vena cava (Figure 4). Although each gland weighs only 3–4 g, it is supplied by three arteries: • a direct branch from the aorta • a branch from the phrenic artery • a branch from the renal artery. The single main vein drains from the hilum of the gland into the nearest available vessel, which is the inferior vena cava on the right or the renal vein on the left. The stubby right suprarenal vein, coming directly from the inferior vena cava, presents the most
Development The thyroid develops from a bud that pushes out from the floor of the pharynx before descending to its definitive position in the neck (Figure 3). It normally loses all connection with its origin, though traces remain in the form of the foramen caecum at the junction of the middle and posterior thirds of the tongue, and the inconstant pyramidal lobe on the isthmus. The development of the thyroid explains the rare occurrence of a lingual thyroid, in which all or a part of the gland remains as SURGERY
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© 2003 The Medicine Publishing Company Ltd
ENDOCRINE
Kidneys and suprarenal glands The left suprarenal gland is said to look like a workman’s cap, while the right is the shape of a top hat. Note especially the short right and long left suprarenal vein Aorta Inferior vena cava Right suprarenal vein
Left suprarenal vein
Gonadal veins
4
dangerous feature in performing an adrenalectomy—the trainee surgeon should always choose the easier left side and leave the right to a more experienced colleague. The suprarenal gland comprises a cortex and medulla, which represent two developmentally and functionally independent endocrine glands in the same anatomical structure. The medulla is derived from the neural crest (ectoderm), cells from which also give rise to the sympathetic ganglia. The cortex, on the other hand, is derived from the mesoderm. The suprarenal medulla receives preganglionic sympathetic fibres from the greater splanchnic nerve and secretes adrenaline and noradrenaline. The cortex secretes the adrenocortical hormones.
FURTHER READING Ellis H. Clinical anatomy. 9th edition. Oxford: Blackwell Science, 1997.
ACKNOWLEDGEMENT The figures in this contribution are modified from Ellis, H. Clinical anatomy. 9th edition. Oxford: Blackwell Science, 1997. The figures have been reproduced with the kind permission of the publishers.
SURGERY
291
© 2003 The Medicine Publishing Company Ltd
jugular veins course over the isthmus. The strap muscles stretch and adhere to the thyroid when it enlarges so that, at operation, they may resemble thin layers of fascia. The larynx and trachea lie on the deep aspect of the thyroid, with the pharynx and oesophagus behind and the carotid sheath on either side. Two nerves are found in close relationship to the gland; the recurrent laryngeal nerve lies in the groove between the trachea and oesophagus, and the external branch of the superior laryngeal nerve lies deep to the upper pole and passes to the cricothyroid muscle.
Anatomy of the thyroid, parathyroid and suprarenal (adrenal) glands Harold Ellis
Parathyroid glands Thyroid gland
The parathyroid glands usually lie on the posterior aspect of the thyroid lobes (Figure 2). There are usually 4 parathyroid glands —a superior and inferior on either side—though the numbers vary from 2 to 6. Ninety percent are in close relationship to the thyroid, while 10%, almost invariably the inferior glands, are aberrant. Parathyroid glands are a yellowish-brown colour and each one is about the size of a split pea. The superior parathyroid is more constant in position than the inferior gland. It usually lies at the middle of the posterior border of the lobe of the thyroid, above the level at which the inferior thyroid artery crosses the recurrent laryngeal nerve. The inferior parathyroid is usually situated below the inferior artery near the lower pole of the thyroid gland. The next most common site is within 1 cm of the lower pole of the thyroid gland. Aberrant inferior parathyroids may descend along the inferior thyroid veins in front of the trachea, and may even track into the superior mediastinum in company with thymic tissues, for which there is an embryological explanation. The superior gland
The thyroid gland comprises three distinct regions (Figure 1): • the isthmus, which overlies the second and third rings of the trachea • the lateral lobes, each of which extends from the side of the thyroid cartilage downwards to the sixth tracheal ring • an inconstant pyramidal lobe, which projects upwards from the isthmus, usually on the left side, and represents a remnant of the embryological descent of the thyroid. The gland is enclosed in the pretracheal fascia, covered by the strap muscles and overlapped by the sternocleidomastoids. The anterior
Harold Ellis is Emeritus Professor of Surgery, London University, London, UK. He is Clinical Anatomist in the Division of Anatomy at King’s College London (Guy’s campus), London, UK.
The thyroid gland and its blood supply
Transverse section of the neck through C6 External carotid artery
Investing fascia
Superior thyroid artery and vein
Pretracheal fascia Anterior jugular vein
Internal jugular vein Sternocleidomastoid Sternohyoid Sternothyroid
Middle thyroid vein Inferior thyroid artery
Omohyoid External jugular vein
Thyrocervical trunk Subclavian artery
Inferior thyroid vein C6
Left brachiocephalic (innominate) vein
Carotid sheath (containing common carotid artery, internal jugular vein, and vagus nerve) with sympathetic chain behind
Pre-vertebral fascia
1
SURGERY
289
© 2003 The Medicine Publishing Company Ltd
ENDOCRINE
Posterior aspect of the thyroid gland
Descent of the thyroid, showing possible sites of ectopic thyroid tissue or thyroglossal cysts Inferior constrictor muscle
Lingual thyroid Suprahyoid thyroglossal cyst
Thyroid gland Superior and inferior parathyroid glands Inferior thyroid artery
Track of thyroid descent and of a thyroglossal fistula
Recurrent laryngeal nerve
Thyroglossal cyst or ectopic thyroid
Oesophagus Trachea Pyramidal lobe 2 Retrosternal goitre
derives from the fourth branchial arch. The inferior gland develops from the third arch in company with the thymus. As the thymus descends, the inferior parathyroid is dragged down with it.
3
Blood supply The thyroid gland is supplied by three arteries and drained by three veins: • the superior thyroid artery arises from the external carotid and passes to the upper pole • the inferior thyroid artery arises from the thyrocervical trunk of the first part of the subclavian artery and passes behind the carotid sheath to the posterior aspect of the gland • the thyroidea ima artery is small and inconstant; when present, it arises from the aortic arch or the brachiocephalic artery • the superior thyroid vein drains the upper pole to the internal jugular vein • the middle thyroid vein drains from the lateral side of the gland to the internal jugular • the inferior thyroid veins (there are often several) drain the lower pole to the brachiocephalic veins. Besides these named branches, numerous small vessels pass to the thyroid from the pharynx and trachea so that, even when all the principal vessels are tied, the gland continues to bleed when cut across during a partial thyroidectomy.
a swelling at the tongue base, and the more common occurrence of a thyroglossal cyst or sinus along the pathway of descent. Such a sinus can be dissected from the midline of the neck along the front of the hyoid then backwards through the muscles of the tongue to the foramen caecum. (It is in such intimate contact with it that the centre of the hyoid must be excised during the dissection.) The thyroid may descend beyond its normal position in the neck down into the superior mediastinum (retrosternal goitre), the most common cause of a superior mediastinal mass.
Suprarenal (adrenal) glands The suprarenal glands cap the upper poles of the kidneys and lie against the corresponding crus of the diaphragm. The typical appearance on CT scan is that of an ‘arrow head’, lying against the easily identified crus on each side. The left is related anteriorly to the stomach across the lesser sac, whereas the right lies behind the right lobe of the liver and tucks medially behind the inferior vena cava (Figure 4). Although each gland weighs only 3–4 g, it is supplied by three arteries: • a direct branch from the aorta • a branch from the phrenic artery • a branch from the renal artery. The single main vein drains from the hilum of the gland into the nearest available vessel, which is the inferior vena cava on the right or the renal vein on the left. The stubby right suprarenal vein, coming directly from the inferior vena cava, presents the most
Development The thyroid develops from a bud that pushes out from the floor of the pharynx before descending to its definitive position in the neck (Figure 3). It normally loses all connection with its origin, though traces remain in the form of the foramen caecum at the junction of the middle and posterior thirds of the tongue, and the inconstant pyramidal lobe on the isthmus. The development of the thyroid explains the rare occurrence of a lingual thyroid, in which all or a part of the gland remains as SURGERY
290
© 2003 The Medicine Publishing Company Ltd
ENDOCRINE
Kidneys and suprarenal glands The left suprarenal gland is said to look like a workman’s cap, while the right is the shape of a top hat. Note especially the short right and long left suprarenal vein Aorta Inferior vena cava Right suprarenal vein
Left suprarenal vein
Gonadal veins
4
dangerous feature in performing an adrenalectomy—the trainee surgeon should always choose the easier left side and leave the right to a more experienced colleague. The suprarenal gland comprises a cortex and medulla, which represent two developmentally and functionally independent endocrine glands in the same anatomical structure. The medulla is derived from the neural crest (ectoderm), cells from which also give rise to the sympathetic ganglia. The cortex, on the other hand, is derived from the mesoderm. The suprarenal medulla receives preganglionic sympathetic fibres from the greater splanchnic nerve and secretes adrenaline and noradrenaline. The cortex secretes the adrenocortical hormones.
FURTHER READING Ellis H. Clinical anatomy. 9th edition. Oxford: Blackwell Science, 1997.
ACKNOWLEDGEMENT The figures in this contribution are modified from Ellis, H. Clinical anatomy. 9th edition. Oxford: Blackwell Science, 1997. The figures have been reproduced with the kind permission of the publishers.
SURGERY
291
© 2003 The Medicine Publishing Company Ltd