GROSS ANATOMY OF THE THYROID AND PARATHYROID GLANDS

THYROID GLAND • Is a brownish-red, highly vascular endocrine organ located in the lower

LOCATION OF THE GLAND • Situated on the anterior side of the neck •

Capsule of the gland • Is invested by a thin fibrous capsule (true capsule);

• Has a parenchyma whose cells are arranged into follicles. • These follicles

Structure • Weighs 20 – 25 g in man; Heavier in females than in

LOBES • Is directed upwards and laterally from the level of the 4 th

RELATIONS OF THE THYROID LOBES • Medially: pharynx, oesophagus, trachea, larynx, external laryngeal and

ISTHMUS OF THE GLAND • Median part of thyroid that connects the two lobes

PYRAMIDAL LOBE • May possess a conical extension (in 50 % of the population)

VASCULAR SUPPLY • Blood vessels of thyroid are located largely between its fibrous capsule

• Additional arterial supply to thyroid gland may arise from the thyroidea ima

• Veins of thyroid form a plexus on the surface of the gland

• A plexus of lymph vessels also exists in the fibrous capsule of

• Nerve fibres reach the gland from the sympathetic ganglion • These nerves

CLINICAL ANATOMY • Thyroglossal cysts • Ectopic thyroid • Hyperthyroidism • Goiter- pathological enlargement

PARATHYROID GLANDS • Are minute ovoid yellowishbrown endocrine organs • Are located posterior to

• Are made up of a parenchyma that consists of principal (or chief)

SUPERIOR PARATHYROID • Usually located about midway along the posterior border of thyroid gland.

INFERIOR PARATHYROID • Usually located behind and close to the lower pole of the

NEUROVASCULAR SUPPLY • Arterial supply- inferior thyroid arteries; Collateral circulation reaches the gland via

• Lymphatic vessels drain (along with those of the thyroid gland) into the

CLINICAL “TITBIT” • The posterior aspect of the thyroid is usually preserved during thyroidectomy

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GROSS ANATOMY OF THE THYROID AND PARATHYROID GLANDS DOSUMU O. O

THYROID GLAND • Is a brownish-red, highly vascular endocrine organ located in the lower part of the neck. • Extends from C 5 to T 1 vertebra

LOCATION OF THE GLAND • Situated on the anterior side of the neck • Lies against and around the larynx and trachea • Posteriorly, it reaches the esophagus and carotid sheath

Capsule of the gland • Is invested by a thin fibrous capsule (true capsule); sends septa into the substance of the gland • Also invested (outside fibrous capsule) by the pretracheal layer of deep cervical fascia [false capsule]

• Has a parenchyma whose cells are arranged into follicles. • These follicles produce thyroid hormones (triiodothyronine and tetraiodothyronine or thyroxine)

Structure • Weighs 20 – 25 g in man; Heavier in females than in males, and increases in weight during menstruation and pregnancy • Consists of a median isthmus and two lobes (right and left)

LOBES • Is directed upwards and laterally from the level of the 4 th (or 5 th) tracheal ring below to the oblique line of the thyroid cartilage above • Has a base and an apex; Apex is directed upwards • Has lateral, medial and posterior surfaces

RELATIONS OF THE THYROID LOBES • Medially: pharynx, oesophagus, trachea, larynx, external laryngeal and recurrent laryngeal nerves • Laterally: (superficially) sternohyoid and sternothyroid • Posteriorly: carotid sheath and its contents, parathyroid glands, inferior thyroid artery

ISTHMUS OF THE GLAND • Median part of thyroid that connects the two lobes • Located ventral to the 2 nd and 3 rd tracheal cartilages • Related anteriorly to sternothyroids, sternohyoids, anterior jugular veins and skin

PYRAMIDAL LOBE • May possess a conical extension (in 50 % of the population) – the pyramidal lobe – that projects towards the hyoid. • A fibromuscular band- levator of the thyroid gland, may descend from the hyoid to the pyramidal lobe (or isthmus) of the gland.

VASCULAR SUPPLY • Blood vessels of thyroid are located largely between its fibrous capsule and the pretracheal layer of the deep fascia of the neck • Arterial blood comes from the superior and inferior thyroid arteries (branches of the external carotid artery and thyrocervical trunk resp)

• Additional arterial supply to thyroid gland may arise from the thyroidea ima artery. • This inconstant unpaired artery arises from the aortic arch or brachiocephalic trunk (present only in 10% of people)

• Veins of thyroid form a plexus on the surface of the gland • From the thyroid venous plexusthree veins • Superior • Middle • inferior thyroid veins • The superior and middle thyroid veins drain into the IJV; inferior drains into brachiocephalic

• A plexus of lymph vessels also exists in the fibrous capsule of the gland; this plexus communicates with those in the interlobular connective tissue • Lymph vessels, which emerge from the thyroid gland end in paratracheal, prelaryngeal and deep cervical nodes

• Nerve fibres reach the gland from the sympathetic ganglion • These nerves fibres are however vasomotor and not secretomotor

CLINICAL ANATOMY • Thyroglossal cysts • Ectopic thyroid • Hyperthyroidism • Goiter- pathological enlargement of the gland (enlargement can compress surrounding structures) • In malignancy- partial thyroidectomy is done and the posterior aspect of thyroid is preserved becos of parathyroid glands

Goiter

PARATHYROID GLANDS • Are minute ovoid yellowishbrown endocrine organs • Are located posterior to the thyroid, two on each side (superior and inferior), within the fibrous capsule of thyroid gland

• Are made up of a parenchyma that consists of principal (or chief) cells. • These cells produce parathyroid hormone (control calcium levels in the body)

SUPERIOR PARATHYROID • Usually located about midway along the posterior border of thyroid gland. • Relatively constant in position • A derivative of the 4 th pharyngeal pouch; therefore also called parathyroid IV

INFERIOR PARATHYROID • Usually located behind and close to the lower pole of the lobe of thyroid • More variable in position; may be found in mediastinum, close to thymus. • Derivative of 3 rd pharyngeal pouchalso called parathyroid III

NEUROVASCULAR SUPPLY • Arterial supply- inferior thyroid arteries; Collateral circulation reaches the gland via superior thyroid arteries, thyroid ima artery, and laryngeal, tracheal and oesophageal arteries. • Parathyroid veins drain into the thyroid plexus of veins.

• Lymphatic vessels drain (along with those of the thyroid gland) into the deep cervical and paratracheal lymph nodes. • Nerve fibres reach the gland from the cervical sympathetic ganglia via the plexus around the thyroid arteries. [These fibres are vasomotor (not secretomotor)]

CLINICAL “TITBIT” • The posterior aspect of the thyroid is usually preserved during thyroidectomy so as not to damage the parathyroid • Damage to parathyroid will lead to hypocalcemia leading to tetany (severe muscle twitches and cramps). Without urgent treatment this will result in death because respiratory muscles will be involved.