Thyroid Gland Lecture

Introduction

The thyroid gland is an endocrine gland with a rich blood supply.
Located in the lower part of the front and sides of the neck.
Functions:
- Regulates basal metabolic rate.
- Stimulates somatic and psychic growth.
- Plays a role in calcium metabolism.
Consists of right and left lobes joined by an isthmus.
A pyramidal lobe may project upwards from the isthmus.

Lies against C5 to T1 vertebra, around the upper part of the trachea.
Each lobe extends from the middle of the thyroid cartilage to the 4th or 5th tracheal ring.
The isthmus extends from the 2nd to the 4th tracheal ring.

Lobes: ~5 cm (length) x 2.5 cm (breadth) x 2.5 cm (thickness).
Isthmus: ~1.2 cm x 1.2 cm.
Average weight: 25 grams. Larger in females; increases in size during menstruation and pregnancy.

True Capsule: Peripheral condensation of connective tissues; contains a dense capillary plexus.
False Capsule: Derived from the pretracheal layer of deep cervical fascia; connects to cricoid cartilage via Barry's ligament.

Each lobe (right and left) has:
- An apex: directed upward and slightly laterally.
- A base: at the level of the 4th or 5th tracheal ring.
- Three surfaces: lateral, medial, posterolateral.
- Two borders: anterior and posterior.
Isthmus: Connects lower parts of the lobes, with anterior and posterior surfaces, and superior and inferior borders.

Arterial Supply:
- Superior Thyroid Artery: First anterior branch of external carotid artery; divides into anterior and posterior branches.
- Inferior Thyroid Artery: Branch of thyrocervical trunk from subclavian artery; divides into 4-5 glandular branches.
- Occasionally, the thyroid is supplied by a thyroidea ima artery.
Venous Drainage:
- Superior, middle, and inferior thyroid veins.
- Superior vein drains into internal jugular vein.
- Middle vein also drains into the internal jugular vein.
- Inferior veins form a plexus and drain into the left brachiocephalic vein.

Lymph from upper part drains into upper deep cervical lymph nodes, either directly or via prelaryngeal nodes.
Lymph from lower part drains into lower deep cervical lymph nodes, and through pretracheal and paratracheal nodes.

Derived mainly from the middle cervical ganglion and partly from superior and inferior cervical ganglia (vasoconstrictor nerves).

Two types of secretory cells:
- Follicular cells: Line the follicles; secrete triiodothyronine and thyroxine (stimulate BMR and growth).
- Parafollicular cells: Lie between follicles; secrete thyrocalcitonin (promotes calcium deposition; induces hypocalcemia).

Thyroid swelling: Palpated from behind, known as goiter.
Thyroidectomy: Removal with true capsule may be necessary in hyperthyroidism.
Hypothyroidism: Causes cretinism in infants and myxedema in adults.
- Superior thyroid artery is ligated near the gland during thyroidectomy to protect the external laryngeal nerve.