The thyroid is a bilobed endocrine organ situated on either side of the trachea and oesophagus. The lobes are joined anteriorly by an isthmus extending over the trachea. Each lobe is about 5 cm in length and extends from the oblique line of the thyroid cartilage to the sixth tracheal ring. The gland’s relationship to other neck structures is indicated in Figure 17.1. It is invested by the pretracheal fascia, which is firmly attached posteriorly to the second to fourth tracheal rings. For this reason, the gland and tumours arising from it characteristically move with the larynx on swallowing.

Fig. 17.1 The anterior and posterior relationships of the thyroid.

The gland produces thyroxine under the control of thyroid stimulating hormone (TSH) secreted by the pituitary. It also contains neuroendocrine parafollicular cells, which produce calcitonin. The thyroid is derived embryologically as a downgrowth from the base of the tongue. A tubular evagination of endodermally derived cells, the thyroglossal duct, extends inferiorly in front of the laryngeal cartilage and the trachea. The distal end proliferates, forming the thyroid lobes and the path of descent should be obliterated. The calcitonin secreting cells are thought to arise as a separate contribution to the embryonic thyroid gland from the fourth and fifth pharyngeal pouches (ultimobranchial body).

Histologically, the gland consists of numerous follicles (Fig. 17.2), which are the functional units capable of synthesising, storing and secreting triiodothyronine and tetraiodothyronine (T₃ and T₄); hormones having a wide range of actions stimulating metabolism. The follicles are spheroidal structures lined by a single layer of cuboidal follicular cells. The cells have microvillous processes embedded in the central store of thyroglobulin or colloid, which is a large iodinated glycoprotein from which T₃ and T₄ are subsequently split after endocytosis. In the euthyroid state the follicles vary in size but average 200 μm in diameter. Secretion of thyroid hormones takes place directly into the rich network of capillaries seen in this vascular organ. Longstanding stored thyroglobulin may accumulate calcium oxalate crystals and ageing follicular cells accumulate lipofuscin.

Fig. 17.2 Normal thyroid histology (H&E).

The parafollicular or C cells are part of the diffuse neuroendocrine system. Although termed parafollicular, these cells are incorporated into follicles. They secrete calcitonin. This hormone has a hypocalcaemic action but its physiological importance in man is unclear. The C cells are immunoreactive to other peptides, including somatostatin, and it is possible that these substances are involved in local paracrine control of T₃ and T₄ production and secretion. These cells are extremely difficult to distinguish from follicular cells using conventional histological stains. The cells are slightly larger, paler and spindle or polyhedral in shape with a faintly granular cytoplasm. They are preferentially localised in the thyroid to the central regions of the lateral lobes and are particularly seen in proximity to solid cell nests, which are thought to be ultimobranchial body remnants.

Fine needle aspiration (FNA) of the thyroid was documented in the Martin and Ellis paper of 1934.¹ It was further developed in papers by Tempka et al. and Piaggio-Blanco et al. in 1948 (cited in Grunze and Spriggs²) and the use of the technique was established subsequently by Scandinavian workers.^3,4 FNA is now recognised to be the first-line investigation for a solitary or dominant thyroid nodule, has a valuable role in the diagnosis of the diffuse non-toxic goitre and can be used to confirm the diagnosis of clinically obvious malignancy, enabling the separation of treatable lymphomas from poor prognosis anaplastic carcinomas. FNA superseded core biopsy of the thyroid. Core biopsy was associated with an increased risk of patient discomfort and complications with little difference in diagnostic value. Modern narrow-bore single action needle devices are believed to be safer and may have a role in selected patients who have had previous inadequate needle aspiration or where FNA findings are discordant with clinical findings.⁵ At present the thyroid is one of the organs most frequently sampled by means of FNA.⁶

The most frequent indication for thyroid FNA is in the evaluation of a solitary thyroid nodule or dominant nodule in the context of a nodular goitre. Prior to consideration of needle aspiration, the history, examination, biochemical and imaging findings need to be considered. Findings of significance for possible thyroid malignancy include a family history of thyroid cancer or adrenal phaeochromocytoma, previous head and neck irradiation, rapid growth, hardness or adherence of the lump to surrounding structures and the presence of associated lymphadenopathy. The minimum piece of biochemical data needed is the TSH level. Serological information and thyroid autoantibody levels can also be helpful in some circumstances. Imaging would usually be by ultrasound. If the TSH level is depressed suggesting thyroid overactivity, a radionuclide scan may be helpful. If the nodule is ‘hot’ indicating localised increased thyroid hormone production, FNA can be avoided as the incidence of malignancy is very low so long as there are no suspicious ultrasound or clinical findings.⁷

Thyroid nodules are now frequently first detected during the course of an imaging study. The management of these ‘incidentalomas’ depends on the type of imaging and in the case of ultrasound detection on the detailed high resolution characteristics. Focal nodules detected by fluorodeoxyglucose-positron emission tomography, FDG-PET scans, have a significant risk of malignancy. They are often primary thyroid cancers even when scanning is for staging another malignancy and warrant FNA.^7,8 Focal hot nodules detected on technetium-99m methoxyisobutylisonitrile scintigraphy, sestamibi scans and confirmed to be discrete on ultrasound require aspiration as they are frequently neoplasms rather than functional nodules.⁷ Nodules first detected sonographically are frequently small. Modern high resolution and Doppler ultrasound has allowed the definition of suspicious features over and above description as solid or cystic and whether there is associated lymphadenopathy. These features include microcalcifications, the irregularity of the nodule margin and intra-lesional vascularity. Lesions of any size with suspicious features should be aspirated. Some of these lesions may be microcarcinomas of uncertain clinical significance but diagnosis and surgical removal is appropriate as a minority behave aggressively. Incidental ultrasonically detected thyroid nodules lacking suspicious radiological or clinical features and less than 10 mm in diameter are usually not aspirated as they have a low risk of malignancy. Larger nodules may be aspirated unless they are clearly cystic and ultrasound follow-up is feasible. Thyroid nodules are seen in at least 16% of patients undergoing neck CT or MRI. They do not automatically warrant needle aspiration. They should be evaluated by ultrasound and the decision to proceed to FNA should be made by the clinician on the basis of those findings.⁷

FNA using palpation alone for guidance is more rapid and less expensive than ultrasound guidance. It is particularly applicable to larger discrete nodules that are clearly within the thyroid. The ultrasonic features of these lesions give useful information as to their nature and allow assessment of the rest of the thyroid and local lymph nodes. If ultrasound is to be carried out then ultrasound guidance should be considered. It allows accurate placement of the needle in less well circumscribed or small nodules, it allows preferential sampling of solid areas in mixed solid-cystic abnormalities, is useful where previous palpation guided FNA has failed, where the anatomy is distorted by prior surgery, where lesions are close to major blood vessels or if the patient has a bleeding diathesis. While ultrasound guidance has advantages in certain circumstances, the adequacy and accuracy of thyroid FNA in large unselected series may be more dependant on the skills and technique of the operator rather than the mode of guidance.⁹

Aspiration by the pathologist with immediate staining and interpretation allows the preparation of optimal specimens and the best appreciation of the clinical history, examination findings and the results of biochemical, serological and imaging data. The procedure should be preceded by clinical examination of the neck from the front and behind the seated patient. The differential diagnostic possibilities of lumps in the neck should be considered (Table 17.1). Patient consent should be obtained. This may be verbal or written depending on local or national policies. The FNA procedure, risks and complications, particularly haematoma formation, should be described. The possibility of a non-contributory, false negative or false positive result should be mentioned.

Table 17.1 Fine needle aspiration of lumps in the neck

Thyroid FNA should be carried out with the patient lying flat, positioned with a pillow beneath the shoulders and neck. This enables the head to fall back in a relaxed position, which separates the sternomastoid muscles, uncovering more of the lateral lobes of the thyroid. Patients will need reassurance as needle aspiration of the neck is one of the more alarming sites for aspiration cytology. They should also be asked not to speak or swallow during the procedure to avoid movement of the gland. A wipe with an alcohol swab is usually sufficient skin cleansing. Routine local anaesthetic injection is not recommended. When aspiration is performed in children topical anaesthetic cream is valuable but its use requires forethought, as the cream must be applied under an occlusive dressing for at least an hour to ensure anaesthesia. The vascularity of the thyroid means that a 23, 25 or 27 gauge bevelled needle should be used. For the majority of thyroid lesions at most only three passes of the needle in a single plane should be carried out; persisting beyond this results in blood contamination of the sample and the dilution or loss of diagnostic features. An exception is in sclerosing malignancies where multiple passes may be necessary to obtain sufficient material. Traversing the sternomastoid should be avoided as it is painful, obscures the depth of the lesion and muscle spasm makes directional control of the needle difficult. Use of a syringe holder allows comfortable single-handed operation of the syringe, freeing the other hand to localise the target.¹⁰ A syringe holder is particularly useful in cystic lesions where the fluid can be drained in a single needle pass.

With ultrasound guided FNA, use of plastic tubing between the needle and syringe allows an assistant to operate the syringe facilitating manipulation of the ultrasound probe. Ultrasonic localisation of the lesion and needle usually results in more bleeding and delay in preparation of slides which diminishes the quality of the specimen. Ultrasound guidance should be restricted to cases where it is advantageous for lesion localisation. The operator should not allow ultrasound gel to contaminate the specimen.⁵

Needle puncture without the use of an aspirating syringe, the capillary technique, may be advantageous for both palpation and ultrasound guided FNA. Holding the needle alone allows fine and controlled passes. Numerous studies have shown that this technique is particularly valid in the thyroid^11–13 and achieves adequate specimens at least as often as conventional aspiration. In the experience of the author and other workers, blood contamination is reduced but the yield of diagnostic material can be reduced.¹⁴ The technique does not facilitate the diagnostic and therapeutic aspiration of cysts in the thyroid but is particularly recommended where previous aspirates have been heavily blood contaminated.

The careful sampling necessary in the thyroid, because of its vascularity, means that at least two aspirates should be taken at any one time from lesions to reduce the risk of false negative diagnosis. Larger nodules should be aspirated in different areas. In the author’s practice, a minimum of four aspirates are taken during the course of initial presentation and subsequent clinical follow-up before a lesion is assumed to be benign and the patient discharged. Where a thyroid tumour coexists with regional adenopathy, both the thyroid and lymph node should be aspirated to rule out coexisting pathology and to allow preoperative staging in the case of thyroid malignancy. Following aspiration, manual pressure should be applied to the site for approximately 1 minute. After the procedure the patient should be advised to sit up slowly and rest for a while before getting up. Most patients experience a little dizziness after having the neck extended backwards during the FNA procedure.

There are no absolute contraindications to needle aspiration of the thyroid in cooperative patients. Overall FNA of the thyroid is a very safe procedure. The main risk is haematoma formation causing tracheal compression in those with large goitres or malignant tumours. There are a few reported cases of acute airway obstruction requiring surgical decompression after thyroid FNA.²³ Care should be exercised in those who are anticoagulated using the smallest needle calibre possible with accurate and controlled needle passes. Clotting status should be checked prior to neck aspiration in those taking therapeutic doses of warfarin or heparin. FNA is usually possible in those taking standard doses of aspirin, non-steroidal anti-inflammatory drugs, antiplatelet drugs and prophylactic doses of heparin. Where it is safe to do so it would be preferable to suspend heparin treatment at least 8 h and antiplatelet therapy 3–5 days prior to aspiration. When there is a particular risk of haemorrhage, patients should be warned to seek medical attention if there is swelling or persisting pain after aspiration. Ultrasound guidance can facilitate the avoidance of larger vessels. Puncture of the carotid requires the aspirator to occlude the puncture site for 5 minutes. Puncture of the trachea may lead to transient coughing. Temporary laryngeal nerve paresis has been recorded post-aspiration,²⁴ as has haemorrhagic necrosis of thyroid tumours, particularly adenomas.²⁵ Needle track implantation by a thyroid malignancy is extraordinarily rare.^26,27 Infection following FNA is also rare.²⁸ Worrisome histological alterations such as regenerative nuclear changes, vascular proliferations, metaplasias and capsular pseudo-invasion may occur in approximately 10% of thyroid excisions following FNA but awareness of these artefacts by histopathologists avoids misdiagnosis. Cytopathologists need to be aware that reaspiration within a few weeks of a previous aspirate can show changes resulting from haematoma formation and healing.²⁹

Aspiration of normal thyroid tissue yields colloid and sheets of follicular cells together with similar dissociated cells (Fig. 17.3). The colloid stains blue/mauve with MGG or the Diff-Quik stain and is seen as a wash of background colour. The Papanicolaou method does not stain colloid well. If the aspirate is contaminated by blood, the diluted colloid may be difficult to distinguish from serum. Follicular cells are relatively small with regular round or slightly oval nuclei of a similar size to those of lymphocytes. The chromatin pattern is even and single small nucleoli are just discernible. Where aspirated in a sheet, the nuclei appear evenly spaced in a honeycomb pattern, each with a small amount of pale, poorly defined, cytoplasm. The cytoplasm of dissociated cells is disrupted and the cells usually appear as bare nuclei. Occasionally whole follicles may be aspirated and these appear as pseudo-giant cells in three- dimensional clusters. The central colloid is usually inapparent (Fig. 17.4).

Fig. 17.3 Sheet of normal thyroid follicular cells with a background ‘wash’ of colloid (MGG).

Fig. 17.4 An intact follicle forming a pseudo-giant cell (MGG).

The follicular cells may contain deep blue intracytoplasmic paravacuolar granules (Fig. 17.5). These consist of lysosomal accumulations of lipofuscin and haemosiderin. These granules can occur in normal thyroid but are more frequently seen in functional pathology, such as in a multinodular goitre where there are cystic and haemorrhagic changes. They also occur occasionally in neoplasms.³⁰ Other pigments may accumulate in the thyroid and prolonged use of minocycline for acne results in a black thyroid with accumulation of lipofuscin and a pigment derived from the breakdown of the antibiotic.³¹

Fig. 17.5 Paravacuolar granules in follicular cells (MGG).

Extraneous cells can be obtained by the passage of the needle through the strap muscles of the neck or sternomastoid muscle. Striated muscle fibres stain a characteristic deep blue colour and cross-striations can be seen. Puncture of the trachea yields respiratory epithelial cells, mucus and occasionally fragments of cartilage. Adipose tissue is not usually obtained except in the very obese and its presence should raise the possibility of a lipoma of the neck,³² although there are other possibilities including aspiration of a thyrolipoma³³ and adipose metaplasia in a nodular goitre.