Thyroid gland importance, Thyroid function tests, Complications of thyroidectomy

The thyroid gland is a small, butterfly-shaped endocrine gland located at the front of the neck, just below the Adam’s apple. It produces thyroid hormones that regulate the body’s metabolism.

Thyroid gland

1. The thyroid gland originates as a primitive bud from the median pharyngeal wall, at the level that will be in adult life the junction between the anterior two-thirds of the tongue and its posterior third, or foramen cecum.
2. It then migrates with the help of the thyroglossal duct (TG) system to its final position, the lower part of the front of the neck.
3. This TG duct then obliterates and disappears.
4. The fourth branchial arch contributes to the embryology of the thyroid gland through the ultimobranchial body.

• This is a population of C cells migrating out of the neural crest. They are so-called because they secrete calcitonin; a calcium-lowering agent in higher primates but not in humans.
• They are also known as parafollicular cells as they lie between the thyroid follicles.
• The neural crest origin of the C cells sheds some light on the well-known association between the medullary thyroid carcinoma (MTC), arising from the C cells, and many other tumors, the most popular of which is the MEN II syndrome.
• Obviously, MTC is not TSH dependent.

Congenital anomalies

1. Agenesis of the thyroid gland.
2. Lingual thyroid gland, partly or completely.
3. Median ectopic thyroid tissue.
4. Thyroglossal duct cyst.
5. Combination of numbers 3 & 4.
6. Ectopic parathyroid glands, especially the lower ones.

Morphology

The thyroid gland looks like a butterfly and consists of two lobes, an isthmus, and a pyramidal lobe. It weighs 25-30 gm. in average, and each lobe measures 5-6 cm long. Sometimes the middle of the thyroid lobe carries a lateral hump well known as Zuckerkandi nodule. Its surgical importance is that it usually hides the RLN behind it, making its dissection a bit more difficult.

It overrides the trachea, where each lobe extends from the oblique line of the thyroid cartilage above to the sixth tracheal ring below. The isthmus overlies tracheal rings number 2 to 4. Laterally, each lobe abuts the carotid sheath.

Arterial supply

The thyroid gland receives its blood supply from many sources.

Superior thyroid artery:

The first branch of the external carotid artery. It enters the gland through its superior pole, where it divides into at least two branches, the anterior and posterior branches. The posterior one contributes a little to the blood supply of the ipsilateral superior parathyroid gland. The superior thyroid artery is accompanied by the superior thyroid vein and the external laryngeal nerve.

Inferior thyroid artery

• One of the three branches coming out of the thyrocervical trunk, which arises from the first part of the subclavian artery.
• It enters the gland through its lateral aspect after giving branches to both ipsilateral parathyroid glands representing their main arterial supply.
• The ITA is closely related to the recurrent laryngeal nerve where the artery is superficial to the nerve in majority of the cases.

Some esophageal and tracheal branches of the corresponding plexuses share in supplying the gland with arterial blood through its deep surface. In a minority of people, the thyroid gland also receives a thyroideaima artery that arises from the aortic arch and enters the gland through the lower border of its isthmus.

Venous drainage

The venous drainage of the thyroid gland does not match exactly its arterial supply.

• The upper part of each lobe is drained through the Superior thyroid vein, which is a tributary of the internal jugular vein. It is accompanied by the superior thyroid artery and the external laryngeal nerve.
• The lower part of each lobe is drained through several veins called inferior thyroid veins that are tributaries of the left innominate vein. The inferior thyroid veins may be closely related to the recurrent laryngeal nerve through its ascent towards the neck.

Some esophageal and tracheal branches of the corresponding plexuses share in draining the gland through its deep surface. In a minority of people, the thyroid gland also has a thyroideaima vein that arises from the lower border of its isthmus and enters the left innominate vein.

Lymphatic drainage

The thyroid gland is drained through a complex network of lymphatics that eventually carry the lymphatic waste to the cervical lymph nodes. Understanding the novel work of the Memorial Sloan Kettering Cancer Center (MSKCC) of New York as regards the distribution of the cervical LNs into specific numbered zones, is crucial to the proper management of malignancies, not only of the thyroid gland, but of all tumors of the head and neck region.

The neck nodes were distributed into six areas in addition to a seventh superior mediastinal area. They are universally known as areas 1 to VII. Like any other organ, the thyroid gland, drains its lymph in a specific order.

The first echelon lymph nodes are present in areas VI & VII, which then drain into areas III, IV, V, II, and lastly I. This order is not obeyed aIl the time, as the location of the tumor in the thyroid lobe affects the order of lymph nodes that become affected by malignant cells.

Nerve supply

• Parasympathetic nerve supply: Vagus nerve
• Sympathetic nerve supply: superior, middle, and inferior sympathetic ganglia of the sympathetic trunk. These small nerves enter the gland along blood vessels.

Fasciae, muscles, and nerves

The thyroid gland is encased within fasciae, protected by muscles, and closely related to important nerves. In contrast to other body regions, the neck is characterized by having three deep fasciae. These are the investing layer of the deep fascia, the pre-tracheal fascia, and the pre-vertebral fascia.

The pre-tracheal fascia:

• It is the one that directly envelopes the thyroid gland.
• It stretches between the thyroid cartilage above and the Aortic arch below.
• Being contained within it, the thyroid gland moves with deglutition.
• It is also a helping factor in retrosternal extension of large goiters, as it guides the gland towards the superior mediastinum, being attached to the aortic arch.
• The posteromedial portion of it is very tough and anchors the corresponding part of the thyroid lobe into the trachea. It is called Berry’s ligament. It hides the RLN behind it just before the entry point of the latter into the larynx.
• The Berry’s ligament carries a small artery and vein within it & therefore should be carefully controlled. During total thyroidectomy, to avoid bleeding in an area that is extremely close to the genu of the RLN.

Muscles

Muscles related to the thyroid gland are the strap muscles. These are four muscles stratified into two strata, two superficial & two deep. It is a bit difficult to remember their exact order. order. But two simple rules can make this job much easier.

1. The first is that the longer two are superficial and the shorter two are deep.
2. The second is that the neck structures are always named from down upwards.

Applying these two simple rules, makes the four muscles easily remembered. These are omohyoid, sternohyoid, sternothyroid & thyrohyoid. The sternothyroid muscle is the direct cover sheet for the thyroid lobe and is the one that is sometimes cut to deliver a huge lobe or excised due to its direct malignant invasion by a thyroid neoplasm.

The strap muscles are also called the extrinsic laryngeal muscles because they move the larynx. They are all supplied by the ansa cervicalis nerve, which enters them low in the neck. The intrinsic laryngeal muscles, on the other hand, are those that move the vocal folds. They all lie inside the larynx and are supplied by the recurrent laryngeal nerve except the crico-thyroid muscle.

It is the only intrinsic laryngeal muscle that lies outside the larynx and is supplied by the external laryngeal nerve. It is the tensor of the vocal folds, while all the other intrinsic laryngeal muscles are responsible for their adduction and abduction.

Nerves

The nerves that have already been mentioned are those that concern the thyroid surgeon. The external laryngeal nerve is a branch of the superior laryngeal nerve, which is in turn a branch of the vagus nerve.

• It is a motor nerve that supplies the cricothyroid muscle: the vocal folds’ tensor.
• It is located in the Joll’s triangle.
• Its injury usually occurs while ligating the superior thyroid vessels, which results in paralysis of the ipsilateral cricothyroid muscle and consequently the patient loses three characteristics of his voice.
• He gets easy fatigability of his voice, loses the ability to produce high-pitched sounds (female singing registers), as well as the high-intensity sounds.

The internal laryngeal nerve is also a branch of the superior laryngeal nerve and is a pure sensory nerve.

• It supplies the interior of the larynx down to the level of the vocal folds.
• Again, the recurrent (inferior) laryngeal nerve (RLN) is a branch of the vagus nerve. On the right side, it arises at the level of the root of the neck and winds around the right subclavian artery.
• On the left side, it arises in the superior mediastinum & then winds around the aortic arch and ascends to the neck. In the neck, it lies in the Beahr’s triangle where it runs in the tracheo-esophageal groove.
• In the groove, it runs deep to the branches of the inferior thyroid artery in most cases. Sometimes, it runs between the branches and is rarely Superficial to the main artery and its branches. On the right side and in less than 2% of the population, the nerve arises in the neck and is thus termed non-recurrent recurrent laryngeal nerve.
• All the reported cases with a non-recurrent recurrent laryngeal nerve on the left side of the neck had a situs inversus scenario!!!
• The RLN is a mixed nerve. It is the motor to the intrinsic laryngeal muscles except the cricothyroid muscle. It is sensory to the interior of the larynx from the level of the vocal folds downwards, including the vocal folds themselves.
• Injury of the recurrent laryngeal nerve leads to different patterns of paralysis of the adductors and abductors of the vocal folds. This in turn leads to an abnormally wide or the more dangerous abnormally narrow Rima glottidis.
• The patient suffers from one or more of a variety of complaints. These include choking, aspiration, inability to cough, hoarseness of voice, dyspnea on rest &/or effort, aphonia, and stridor.

Parathyroid glands

• The upper parathyroid glands arise from the fourth branchial arch.
• The lower parathyroid glands, together with the thymus, arise from the third branchial arch.
• They have to migrate a distance longer than the upper ones, so they are more likely to be ectopic.
• Their migration together with the thymus makes the superior mediastinum one of the commonest locations for such an ectopic parathyroid gland.
• Most of the blood supply to the parathyroid glands comes from the inferior thyroid artery.

Micro-anatomy

The gland has a thin fibrous capsule from which septae divide the gland into lobes and lobules. The lobule is composed of follicles, each of which consists of follicular cells arranged in a circular pattern around the thyroglobulin-filled center. In between the follicles are distributed the parafollicular C cells, lymphocytes, blood vessels, and septa

Applied physiology

• The thyroid gland is an endocrine gland that secretes thyroxine hormone in two forms, well known as T3 & T4, directly into the blood. 
• It is under the feedback control of the pituitary gland through the thyroid-stimulating hormone (TSH).
• The pituitary gland, in turn, is under the feedback control of the hypothalamus through the thyrotropin-releasing hormone (TRH).

The thyroid gland

• Concentrates inorganic iodine from the blood against its concentration, which is about 100 times more inside the thyroid cells than in the blood.
• Then, through a number of chemical reactions, the iodide salt is organified into organic iodine, then linked to tyrosine, forming monoiodotyrosine & diiodotyrosine.
• then coupled forming T2, T3, & T4.
• The T2 has no metabolic pathway, so it is degraded once more.
• The other two hormones are combined to globulins, forming thyroglobulin or Colloid material, stored in the center of each follicle.

These chemical reactions are blocked at different points by certain foods as cabbage, cauliflower & others, but more important, by anti-thyroid drugs.

When needed, the thyroid gland releases its hormones into the blood after breaking the thyroglobulin into its two components through a step called proteolysis. When the free hormones are released into the blood, nearly 99% of them become again attached to plasma proteins, leaving only 1% as free hormones, (FT3 & FT4), exerting all the metabolic effects of the thyroid hormones.

It is important to know that the proteolysis step çan be blocked by an excess of iodine intake as happens intentionally in the preoperative preparation of thyrotoxic patients, and as a side effect of the prolonged intake of lithium, which is a constituent of mood stabilizing agents and anti-psychotic medications.

Under normal physiological conditions, the three of these hormones TSH, T34 & T4, are all in normal ranges. In hyperthyroid states, T3 & T4 rise significantly in the blood, and TSH becomes almost undetectable. In hypothyroid states, T3 & 14 declines in blood, and so TSH increases significantly.

Since thyroid hormones are bound to plasma proteins, their total estimation is not accurate to diagnose the actual thyroid status. So, no one should measure T3 & T4 any longer. Instead, free portions of both hormones are better detected in the serum. These are known as FT3 & FT4.

Investigations

Laboratory: Thyroid function tests

• TSH
• FT3
• FT4
• Anti-thyroglobulin antibodies.
• Anti-peroxidase antibodies.
• TSH receptor antibodies (Trab).

Imaging

• Plain X-ray neck & chest.
• Barium swallow.
• Ultrasound neck (the most important).
• CT and MRI neck.

Radioisotope studies

• Thyroid scanning.
• Whole body scanning (WBS).

Others

• Biopsy (FNAC) (Extremely important in diagnosis if sonographically indicated.
• Laryngoscopy. Important for the assessment of the vocal cords.

Routine investigations

• TSH, FT3 & FT4
• Thyroid auto-antibodies, if Hashimoto’s thyroiditis is to be excluded.
• US FNAC (Fine-needle aspiration cytology).
• Others: tailored according to each case.

Surgical treatment

Indications of thyroidectomy

• Cosmesis.
• Compression (Dyspnea-Dysphagia).
• Complications:
• Hemorrhage.
• Retrosternal extension.
• Cancer or its suspicion.

Types of thyroidectomy (depending on extend of removal)

• Total thyroidectomy.
• Near-total thyroidectomy
• Hemi-thyroidectomy.
• Isthmectomy.

Types of thyroidectomy (Depending on time of surgery):

• Elective thyroidectomy.
• Prophylactic thyroidectomy.
• Incidental thyroidectomy.
• Emergency thyroidectomy.

Complications of thyroidectomy:

Early

Stridor:

• RLN damage
• Laryngeal oedema
• Laryngeal spasm
• Tracheomalacia
• Hematoma
• Tracheal injury
• Esophageal injury
• Thyroid crisis
• Vomit aspiration
• Pain
• Dysphagia

Delayed:

• Hematoma (usually subcutaneous).
• Seroma
• Infection; Cellulitis – infected collection.
• Hypocalcemia
• Ugly scar
• Voice changes
• Stridor

Postoperative days

• Hospital stay, how long? The standard is 24 hours.
• Hospital medications: mainly pain killers.
• Drain removal, when? The standard is 1-3 days
• Take home medications: The standard are pain killers, calcium, and Vitamin D replacement.
• Postoperative instructions.
• Stitch removal, when? The standard is 5-7 days.
• Scar medications.

Follow-up regimen for non-neoplastic goiters

• Must we give replacement therapy? No for hemi removal, yes for total removal.
• When to start replacement therapy? Following issuing of the pathology report.
• How much to replace? The standard is to start with 100 micros daily.
• How frequently to I check my hormonal blood levels? After 2 months, then annually.
• What parameters to check? TSH + FT4.

Follow-up regimen for neoplastic goiters

• TSH + FT4 in addition to:
• Thyroid tumor markers: Thyroglobulin, Anti-thyroglobulin antibodies, Calcitonin, CEA (Carcinoembryonic antigen).
• Neck ultrasound
• RAI WBS, after the radioactive iodine treatment, a whole body scan (post-RAI WBS) is done to identify any thyroid cancer that has spread outside the thyroid bed.

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