Thyroid

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Contents

Introduction

The thyroid gland lies in the anterior triangle of the neck. Thyroid follicular cells arise from the foramen caecum in the tongue, and during embryological development the gland descends in the midline to the level of the larynx, where it buds laterally. A contribution of neural crest cells arising from the fourth branchial cleft and the ultimobranchial body gives rise to the C-cells of the thyroid as well as to a small projection from the postero-lateral surface (the Tubercle of Zuckerkandl). The thyroid comprises two lateral lobes, an isthmus and a pyramid. The function of the follicular cells is to produce, store and release the thyroid hormones thyroxine (T4) and tri-iodothyronine (T3). The function of the C-cells is to produce the calcium-lowering hormone calcitonin.

Diseases affecting the thyroid gland can generally be grouped into disorders of thyroid function (including hypothyroidism and thyrotoxicosis) or disorders affecting thyroid structure (including nodular goitre and thyroid neoplasms). Disorders of both function and structure coexist and, from the surgical point of view, thyroid disease can be considered in the following groups:

  • Disorders of embryological development
  • Benign thyroid nodules
  • Multinodular goitre
  • Thyroid cancer
  • Thyrotoxicosis
  • Thyroiditis

Disorders of the thyroid gland

Disorders of embryological development

Pathology

As the thyroid gland descends during embryological development, remnants of thyroid tissue may be left behind anywhere along the thyroglossal tract. Occasionally some respiratory epithelium may be included in the remnant, resulting in a thyroglossal duct cyst. A non-cystic benign thyroid nodule, or even thyroid cancer, may also develop anywhere along the tract. Embryological descent may also continue into the anterior mediastinum, giving rise to a thyrothymic thyroid remnant.

Clinical presentation

Thyroglossal duct cysts most commonly present either in childhood or adolescence. The usual clinical presentation is that of a midline swelling that moves with protrusion of the tongue. Inflammation may give rise to an abscess or a fistula. Thyrothymic thyroid remnants usually remain asymptomatic unless other pathology, e.g. nodular change, develops - in which case they may present as an intrathoracic goitre.

Investigations

The diagnosis of a thyroglossal duct cyst is usually selfevident on clinical examination. Fine-needle aspiration cytology (FNAC) will confirm the nature of the cyst contents, although this may precipitate inflammation.

Treatment

The only treatment is surgical excision through a skincrease incision. In order to prevent recurrence, the cyst and the thyroglossal tract must be removed, which involves excision of the mid-portion of the hyoid bone and tracing of the tract to its upper limit. Thyrothymic thyroid remnants which are symptomatic require surgical excision.

Benign thyroid nodules

Pathology

Between 30 and 40% of clinically single thyroid nodules will represent a dominant nodule in a multinodular goitre. Of the remainder, the majority will be benign nodules, either simple thyroid cysts, solitary colloid nodules, or benign follicular adenoma. Approximately 7% will be a thyroid cancer and a further small group will represent an area of nodularity within thyroiditis.

Clinical presentation

The most common presentation of a single thyroid nodule is that of an asymptomatic swelling in the neck. The characteristic clinical feature of a thyroid nodule is that it moves on swallowing. Benign thyroid nodules may also present with local pressure symptoms. These include dysphagia from oesophageal pressure, a persistent cough or stridor from tracheal pressure, a hoarse voice from pressure on the recurrent laryngeal nerve, or superior vena cava (SVC) obstruction from a large single nodule obstructing the thoracic inlet. A toxic nodule will present with symptoms of thyrotoxicosis.

Investigations

Thyroid function tests (TFTs), including thyroid stimulating hormone (TSH), free T4 and free T3, should be performed in all patients, especially the elderly, in order to exclude subclinical thyrotoxicosis, or to diagnose a toxic nodule or an autonomously functioning nodule. In the latter case, a suppressed TSH will be seen in association with normal free T4 and free T3 levels. Fine-needle aspiration cytology is the definitive investigation, and the possible cytological reports are categorised in Classification of fine-needle aspiration cytology reports. It is important to note that an atypical report indicates the presence of a follicular neoplasm, but cannot differentiate between a benign follicular adenoma and a follicular carcinoma. This diagnosis can only be made by the finding of either capsular or vascular invasion on histology. Ultrasound scans have limited value in the investigation of thyroid nodules, although FNAC is best performed with ultrasound guidance. The role of nuclear medicine scans is now limited to the investigation of patients with thyrotoxicosis or thyroiditis.

Table 35. Classification of fine-needle aspiration cytology reports
Category Cytology Pathology Interpretation
Inadequate Insufficient cells or excess blood Needs to be repeated
Cystic Cyst fluid and degenerate cells Simple thyroid cyst or colloid cyst Repeat if cyst recurs
Benign (benign follicular pattern) Normal follicular cells and abundant colloid Colloid nodule or macrofollicular adenoma Conservative management if asymptomatic
Atypical (suspicious; atypical follicular pattern; follicular neoplasm) Microfollicles and scant or absent colloid Microfollicular adenoma or follicular carcinoma Surgery required because a follicular carcinoma cannot be excluded
Malignant Cells consistent with the specific histology Papillary, anaplastic or medullary carcinoma, lymphoma Surgery appropriate to the specific type of cancer
Thyroiditis Inflammatory cells and colloid Hashimoto's thyroiditis, de Quervain's thyroiditis Therapy specific to the type of thyroiditis

Treatment

Asymptomatic thyroid nodules that are benign on FNAC do not generally require treatment. Indications for surgery include the presence of obstructive symptoms, thyrotoxicosis, or the finding of either malignancy or atypical changes on FNAC. The minimal surgical procedure is a lobectomy, removing all thyroid tissue on the side of the lesion. Thyroxine suppression is generally ineffective in decreasing the size of single thyroid nodules.

Multinodular goitre

Pathology

Multinodular goitre occurs as the result of repeated cycles of hyperplasia, nodule formation, degeneration and fibrosis occurring throughout the gland. It occurs either in response to iodine deficiency or else, in iodinereplete areas, as the result of intrinsic heterogeneity of TSH receptors. The latter has a high familial incidence. A dominant nodule within a multinodular goitre is most likely to be either a hyperplastic or colloid nodule. However, the incidence of malignancy in a dominant nodule is approximately the same as for a single nodule (7%). Multinodular goitre has a high familial incidence, and is also common in areas where iodine is deficient in the diet.

Clinical presentation

Most multinodular goitres present as an asymptomatic mass in the neck. They may also present with local obstructive symptoms to the trachea, oesophagus, recurrent laryngeal nerve or SVC. Thyrotoxicosis is also common, especially in the elderly with large goitres. An otherwise asymptomatic retrosternal multinodular goitre may present as a mass on a chest X-ray or computed tomography (CT) scan.

Investigations

Thyroid function tests must be performed on all patients. FNAC of the dominant nodule or nodules will exclude malignancy and a CT scan will assess retrosternal extension or tracheal compression. An ultrasound adds little to the clinical examination. Nuclear medicine scan only serves to confirm the presence of multinodular change and has little value.

Treatment

Indications for surgical treatment of a multinodular goitre include the presence of obstructive symptoms, thyrotoxicosis, suspicious or malignant changes on FNAC, a strong family history of thyroid cancer, the presence of retrosternal extension, or a past history of head and neck irradiation. In a young patient with a large multinodular goitre who is otherwise asymptomatic, thyroidectomy may also be considered for cosmetic reasons. The only effective treatment is surgical excision. If surgery is undertaken, total thyroidectomy is the preferred option, because it removes all tissue likely to cause symptoms and avoids the possibility of later recurrence, which is of the order of 30%. Lifelong thyroxine replacement is required after total thyroidectomy.

Thyroid cancer

Pathology

Thyroid cancer can arise from the follicular cells, the C-cells, or other cells such as lymphocytes or stromal cells, each giving rise to a different form of thyroid cancer. Papillary thyroid cancer accounts for about 85% of thyroid cancers, tends to occur in a younger age group (20–40 years), is often multifocal, spreads predominantly to local lymph nodes and has a relatively good prognosis, with 10 year survival rates of at least 90%. Follicular cancer occurs in an older age group (40–60 years), arises as a single tumour, metastasises by the bloodstream, and has a worse prognosis than papillary cancer, with 10 year survival rates around 75%. Anaplastic, or undifferentiated, thyroid cancer occurs in the elderly, often presents as a rapidly enlarging diffuse mass, spreads locally, and has a terrible prognosis, with 5-year survival rates of less than 1%. Malignancy arising in C-cells gives rise to medullary carcinoma of the thyroid. This tumour secretes calcitonin, and may be part of a familial multiple endocrine neoplasia syndrome (MEN IIA), occurring in association with phaeochromocytoma and hyperparathyroidism. Ten-year survival is around 35%. Lymphoma of the thyroid is thought to arise in lymphocytes, often in association with pre-existing Hashimoto's thyroiditis. Miscellaneous thyroid malignancies include squamous cancer, sarcoma and metastases, but are all rare presentations. Follicular-derived thyroid cancer has an association with previous exposure to ionising radiation.

Clinical presentation

Thyroid cancer commonly presents as either a single thyroid nodule or a dominant nodule in a multinodular goitre. Thyroid cancer may also present as metastatic disease, such as bony metastases from follicular cancer, or lymphangitic lung involvement from papillary cancer.

Investigation

Depending on the presentation, the most useful investigation is FNAC, but CT scanning is of value in determining extent of tumour and lymph node involvement. Serum calcitonin levels may be raised in medullary tumours.

Treatment

The treatment of all but very-low-risk differentiated thyroid cancer (follicular or papillary) is total thyroidectomy, with local removal of involved lymph nodes. Subsequent treatment involves the administration of radioactive iodine (131I), which allows for both the detection and ablation of metastatic disease and lifelong thyroxine suppression. Low-risk papillary cancer includes young patients with small, often incidentally discovered tumours. Minimally invasive follicular thyroid cancer with capsular penetration alone is also regarded as a low-risk cancer. Both of these may be adequately treated with lobectomy and long-term thyroxine suppression.

Medullary thyroid cancer requires total thyroidectomy and a central lymph node dissection, with lateral neck dissection and mediastinal clearance for node positive patients. Medullary carcinoma does not respond to radioiodine ablation. Surgery for thyroid lymphoma should be limited to biopsy only, because the tumour responds well to external beam radiotherapy and/or chemotherapy. Surgery should be avoided for anaplastic cancer, although pre-operative multimodal chemoradiotherapy may well make such tumours operable.

Thyrotoxicosis

Pathology

Thyrotoxicosis can occur in diffuse hypersecretory goitre (Graves' disease), toxic multinodular goitre (Plummer's disease), toxic follicular adenoma or in the initial stages of thyroiditis. Rarer causes include a TSH-secreting pituitary tumour or struma ovarii. Graves' disease is an autoimmune condition associated with antibodies to the TSH receptor. Toxic nodular goitre results from autonomous activity in a neoplastic nodule.

Clinical presentation

Thyrotoxicosis will present with signs and symptoms of thyroid overactivity, including tachycardia, heat intolerance, sweating, weight loss and anxiety. In addition, Graves' disease may be associated with exophthalmos and pre-tibial myxoedema. Toxic nodules may also present with local pressure or obstruction.

Investigation

The diagnosis of thyrotoxicosis will be confirmed by TFTs with an elevated free T4 and/or free T3, in association with a suppressed TSH. Clinical examination may well indicate the aetiology, demonstrating a diffuse goitre, a multinodular goitre or a single thyroid nodule. Thyroid nuclear scans will confirm the diagnosis, as well as the aetiology.

Treatment

Initial treatment is to render the patient euthyroid by administration of antithyroid medication in the form of propylthiouracil or carbimazole, both of which prevent coupling of iodotyrosine. Patients with Graves' disease are generally treated with medication for 12–18 months. Alternative treatments are ablation with radioactive iodine, usually reserved for patients more than 40 years of age because of the theoretical teratogenic risk, or total thyroidectomy. Graves' disease is treated by surgery if there is relapse following initial medical treatment (about 50% of the time) or non-compliance with such treatment, or if ophthalmopathy is present, in which case radioiodine ablation is contraindicated. Toxic multinodular goitre and toxic adenoma are best treated surgically once the patient is rendered euthyroid by antithyroid medication.

Thyroiditis

Pathology

Thyroiditis is classified as lymphocytic (Hashimoto's), subacute (de Quervain's), acute (bacterial) or fibrosing (Reidel's). Of these, the two most common are lymphocytic, which is an autoimmune condition often forming part of a spectrum with Graves' disease, and subacute, which is a post-viral phenomenon.

Presentation

Lymphocytic thyroiditis may present with hyperthyroidism (early phase) or hypothyroidism (late phase), or it may present with a nodular or diffuse goitre. Subacute thyroiditis usually presents with an exquisitely tender, enlarged, firm thyroid gland, often with systemic symptoms of headache, malaise and weight loss.

Investigation

Thyroid function tests will determine the level of thyroid activity. A nuclear medicine scan is often diagnostic, showing patchy uptake in lymphocytic thyroiditis and no uptake at all in subacute thyroiditis.

Treatment

Subacute thyroiditis usually responds to high-dose steroids and aspirin therapy, although it may take 3–6 months to fully resolve. Lymphocytic thyroiditis may respond to thyroxine suppression. Surgery may be required for lymphocytic thyroiditis with persistent or suspicious nodules, or for pressure symptoms, although surgery is difficult because of the solid, nonpliable nature of the gland.

Operative management: thyroidectomy

Indications

Thyroidectomy (Timing of suture removal) is indicated for relief of local obstructive symptoms, for the diagnosis and treatment of thyroid cancer, for the control of thyrotoxicosis, or for cosmetic considerations.

Complications of thyroidectomy

The complications of thyroidectomy include all the general complications of any operation, such as bleeding, wound infection, and reaction to the anaesthetic agent. In addition, there are specific complications, including:

  • Damage to the recurrent laryngeal nerves
  • Damage to the external branch of the superior laryngeal nerves
  • Damage to the parathyroid glands

Unilateral recurrent nerve palsy leads to a hoarse voice that, if permanent, can be treated by procedures such as vocal cord medialisation. Bilateral damage may require a tracheostomy because the cords adopt the medial position. If the external branch of the superior laryngeal nerve is affected, the patient may lose the ability to sing, shout or project their voice. Permanent damage to the parathyroid glands will cause permanent hypoparathyroidism. Temporary damage or oedema, which is much more common, will require short-term administration of oral calcium and 1,25-dihydroxyvitamin D for several weeks. These complications are avoided by understanding the surgical anatomy of the thyroid gland (Surgical anatomy of the thyroid gland. The left lobe of the gland is elevated exposing the recurrent laryngeal nerve, the external branch of the superior laryngeal nerve, the Tubercle of Zuckerkandl, and both parathyroid glands, with their blood supply arising from the inferior thyroid artery.) and by the technique of capsular dissection, carefully avoiding the parathyroid glands and their blood supply, and the recurrent laryngeal nerves.

Surgical anatomy of the thyroid gland. The left lobe of the gland is elevated exposing the recurrent laryngeal nerve, the external branch of the superior laryngeal nerve, the Tubercle of Zuckerkandl, and both parathyroid glands, with their blood supply arising from the inferior thyroid artery.

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