The Thyroid Gland

Development
The thyroid gland develops as a median down growth of a column of cells from the pharyngeal floor between the first and second pharyngeal pouches (subsequently marked by the foramen caecum of the tongue). The canalized column becomes the thyroglossal duct which is displaced forwards by the developing hyoid bone and then, below the hyoid, lies slightly to one side, more commonly the left. The duct bifurcates to form the thyroid lobes and a portion of the duct forms the pyramidal lobe (Fig. 162)

Congenital anomalies
A few clinically relevant developmental errors may occur.
Ectopic Thyroid Tissue: Ectopic thyroid tissue may lie anywhere along the line of descent and May in rare instances, be the only thyroid tissue present. An occasional site for this anomaly is in the base of the tongue. Ectopic thyroid tissue can be identified by isotope scanning.
Thyroglossal Cyst: A thyroglossal cyst results from persistence of a portion of the thyroglossal duct. It is the commonest congenital error and presents as an asymptomatic swelling somewhere along the course of the duct, but usually below the level of the hyoid bone. These cysts usually become clinically evident in the late teens, presumably because the lining becomes secretory. There is often thyroid tissue associated with the cyst and, in some cases, this may represent the only thyroid tissue present. This fact must be established before surgical excision is performed to avoid rendering the patient hypothyroid.

Anatomy
The thyroid gland occupies an important position in the centre of the visceral compartment of the neck, lying astride the trachea just above the thoracic inlet. Normally weighing about 25 g. the anatomy of this vital endocrine gland is relevant both to the non-operating clinician and in operative surgery.

The gland has two lobes, shaped roughly like slender pears, hugging the anterolateral aspect
of his cervical trachea from the level of the thyroid cartilage to the 5th or 6th tracheal ring. The right lobe is often larger than the plastered quite firmly to the anterior surface of the trachea, at the level of the 2nd and 3rd tracheal rings. A variable -sized, but usually small, pyramidal lobe arises from the isthmus somewhere along its upper border near the midline. The thyroid gland is covered by fascia and the strap muscles and more laterally, it is tucked under the diverging anterior borders of the sternoclidomastoid muscles.

Anatomy
Clinical beginners often search for the gland too high in the neck. It should be palpated from behind the subject with the middle and index fingers lying just above the sternoclavicular joint across the trachea, as though spreading the converging sternoclidomastoid muscles. Because of its fascial attachments, the gland moves upwards with swallowing and, therefore, it slides under the examining fingers. The normal gland can be felt in thin necks. It is soft and supple and the tracheal rings can be palpated through it.



The Vascular Supply

As would be expected from its endocrine function, the blood supply of the thyroid gland is very rich in the hyperthyroid state, and there may be an enormous increase in the volume of blood circulating through the gland. Each thyroid lobe is supplied by a superior and an inferior thyroid artery and drained by three veins.

The Superior Vascular Pedicle: The superior vascular pedicle contains the superior thyroid artery, which is the first branch of the external jugular vein. The external laryngeal nerve is closely related to this pedicle and is discussed below. The superior vessels enter the upper pole of the gland at its apex with branches to the front and back of the gland. These superior vessels are easily dealt with surgically, because the lose space between the two capsules is developed at the upper pole of the thyroid lobe and a ligature is placed close to the upper pole, to include both vessels and exclude the external laryngeal nerve.

The Inferior Thyroid Artery: The inferior thyroid artery and vein do not relate to each other al all. The artery arises from the thyrocervical trunk, passes behind the carotid sheath and then runs transversely across the space between this and the thyroid gland to enter the deep surface of the gland as several separate branches close to the tracheothyroid groove. These terminal branches of the inferior thyroid artery are uncomfortably close to the recurrent laryngeal nerve and the inferior parathyroid gland and should be surgically shunned. If the inferior thyroid artery is to be ligated, it should be done in its transverse portion medial to the carotid sheath.

The Vascular Supply
  • The Inferior Thyroid Veins: The inferior thyroid veins, of which there are always a few on each side, leave the lower border of the gland and pass through the loose fascial space to join the left brachiocephalic vein. They are fragile and require being ligated single.
  • The Middle Thyroid Veins: The applied anatomy of the middle thyroid vein is important because it is a short, thin-walled vessel, leaving the middle of the gland and directly coursing laterally to pass in front of behind the carotid artery and enter the internal jugular vein. It is the first vessel encountered in thyroidectomy and merits careful ligation when it is met early on during the development of the intercapsular space mentioned above.
  • The Thyroidea Ima Artery: The Thyroidea Ima artery from the brachiocephalic trunk extending in front of the trachea is small and surgically irrelevant.


The Parathyroid Glands
The number of parathyroids very from 2 to 6, but, in 80 percent of cases, there are 4 (2 on each side). The total weight of 4 normal glands is about 140mg.

Embryology: The upper parathyroids arise from the 4th branchial pouch and come to lie in close association with the upper part of the lateral lobes of the thyroid. This position is constant.
The lower parathyroids arise from the 3rd branchial pouch in association with the thymus, and descend with the thymus. Because of this embryological migration they may be found anywhere from the upper pole of the thyroid to the anterior mediastinum.

Anatomy
The glands are the size of a split pea. They are pink or brown in colour, but are frequently covered by fat, making them difficult to recognize.
The superior glands lie on the posterior surface of the middle third of the thyroid, usually above the inferior thyroid artery, but well posterior to this plane. If enlarged they may descend into the posterior mediastinum.
The inferior glands are mostly found on the posterior surface of the lower pole of the thyroid or within 1 cm below the lower pole. The may be higher or lower, occasionally as far down as in the thymus in the anterior mediastinum. They lie in a more anterior plane than the upper glands.
A parathyroid gland located within the surgical false capsule of the thyroid, when diseased, remains in place locally. A gland outside the capsule is often displaced into the posterior mediastinum. Sometimes the parathyroids may be embedded in the thyroid gland.

Blood supply
A special small parathyroid artery supplies each gland. The lower parathyroid artery comes from the inferior thyroid artery and is a guide to the gland if it lies below the lower margin of the thyroid.
The upper parathyroid artery arises from the inferior artery or from an anastomosing artery joining the superior and inferior thyroid arteries and only very occasionally from the superior thyroid artery.
There is a good collateral arterial supply from the tracheal vessels and adequate parathyroid function persists even if all four major thyroid arteries are ligated.

Examination of the Thyroid

Instruction
Examine the thyroid.
Test this patient’s thyroid status.
Look at the neck.

Salient features

l  Introduce yourself to the patient and while shaking hands note whether his or her palms are warm and sweaty.
l  Inspection of the neck:
            Look for the JVP
            Scars of surgery.
            Enlarged cervical lymph nodes.
            Goiter.
l  Palpation (always begin by palpating from behind):
            Seat the patient comfortably.
            While palpating the gland ensures that there is a glass of water to swallow.

Palpate the thyroid and note the following:

Size
Mobility
Texture- simple or nodular
(Solitary or multiple)?
Tenderness.

Pembertion’s sign (on raising the arms above the head patients with retrosternal goiters may develop signs of compression, i.e. suffusion of the face, syncope or giddiness).
l  Palpate cervical lymph nodes.
l  Feel the carotid arteries.
l  Palpate for tracheal deviation.
l  Percuss for retrosternal extension.

Auscultate over the gland for bruit, carotid bruits.
Test sternomastoid function (this muscle may be infiltrated in thyroid malignancies).

l  Thyroid function should then be assessed:
1. Eye signs:
l  Lid lag.
l  Exophthalmos.
l  Lid retraction (sclera visible above the cornea).
l  Extra ocular movements.

2. Hand:
l  Pulse for tachycardia or atrial fibrillation
l  Tremor.
l  Palmer erythema (thyrotoxicosis).
l  Supinator jerks (inverted in hypothyroidism).
l  Proximal weakness in the upper arm.
3. Skin: look for pretibial myxoedema.
4. Elicit the ankle jerks.

Graves ’ disease
Instruction
Look at this patient.
Determine this patient’s thyroid status.

l  Salient features
l  Patient is fidgety and restless.
l  While shaking hands with the patient note the warm sweaty palms.

l  Look for tremor, thyroid acropachy, onycholysis (Plummer’s nails) and palmer erythema.
l  Check pulse (for tachycardia or the irregularly irregular pulse of atrial fibrillation).
l  Comment on proptosis (after looking at the eyes from behind and above).
l  Check for lid lag.
l  Check for scars of previous tarsorrhaphy.
l  Examine the neck for goiter and auscultate over the gland.
l  Mention previous thyroidectomy scar if present.
l  Check the shins for pretibial myxoedema (bilateral pinkish, brown dermal plaques).
l  Test for proximal myopathy.
l   

l  Tell the examiner that you would like to ask the patient about the following:
            - Easy irritability.
            - Weight loss with increased appetite.
            - Frequent defecation.
            - Oligomenorrhoea.
            - Dislike for hot weather.
            - Family history of thyroid disease.

Exophthalmos
Instruction
Examine this patient’s face.
Perform a general examination of this patient.

l  Salient features
l  Prominent eyeballs.
l  Look at the patient’s eyes from behind and above for proptosis.
l  Comment on lid retraction (the sclera above the upper limbus of the cornea will be seen); this is Dalrymple’s sign.

l  Comment on the sclera visible between the lower eyelid and the lower limbus of the cornea (i.e. comment on the exophthalmos). Most patients have bilateral exophthalmos with unilateral prominence.
l  Check for lid lag (ask the patient to follow your finger and then move your finger along the arc of a circle from a point above his or her head to a point below the nose- the movement of the lid lags behind the globe); this is von Graefe’s sign.
l  Check for extraocular movements and comment on the cornea.

l  Look for the following:
            - Signs of thyrotoxicosis
            (Fast pulse rate, tremor, sweating).
            - Goiter (listen for bruit).
            - Post-thyroidectomy scar.

Pretibial Myxoedema
Instruction
Look at this patient’s legs.

Investigation:
l  It is important to confirm the presence of thyrotoxicosis biochemically by more than one test of thyroid function in view of the likely need for prolonged medical treatment or destructive therapy. Serum T3 and T4 are elevated in the majority, but T4 is in the upper part of the normal range and T3 raised (T3 toxicosis) in 5% of patients. In primary thyrotoxicosis, serum, TSH is undetectable at less than 0.05mU/l. Further tests, which may be required to establish the aetiology of thyrotoxicosis, include measurement of TSH receptor antibodies (TRAB, elevated in Graves’ disease, and isotope scanning.

Management:
l  Definitive treatment of thyrotoxicosis depends on the underlying cause, and may include antithyroid drugs, radioactive iodine or surgery. In all patients with thyrotoxicosis a non-selective b adrenoceptor antagonist (b-blocker), such as propranolol (160 mg daily) or nadolo (40-80mg daily), will alleviate but not abolish symptoms within 24-48 hours. Beta-blockers cannot recommended for long-term treatment, but they are extremely useful in the short term, e.g. for patients awaiting hospital consultation.