Thyroid Gland Surgical Anatomy
- The thyroid gland is present in the neck, enclosed by pre tracheal fascia which is a part of the deep cervical fascia.
- It has a right and left lobe joined by an isthmus which is in front of the 2nd, 3rd and 4th tracheal rings.
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- It weighs about 20–25 g. A projection from the isthmus usually on the left side is called a pyramidal lobe.
- It is attached to the hyoid bone by a fibrous band or muscle fibres called levator glandular thyroid.
- Suspensory ligament of Berry: This pair of strong condensed connective tissue binds the gland firmly to each side of the cricoid cartilage.
- Pretracheal fascia, which is a part of deep cervical fascia splits to invest the gland. These few structures are responsible for the thyroid moving with deglutition.
Thyroid Gland Development
- It develops from the median down growth (midline diverticulum) of a column of cells from the pharyngeal floor between the first and second pharyngeal pouches.
- By 6 weeks time the central column, which becomes the thyroglossal duct, gets reabsorbed.
- The duct bifurcates to form thyroid lobes.
- The pyramidal lobe is formed by a portion of the duct.
Thyroid Gland Arterial Supply
- The superior thyroid artery is a branch of the external carotid artery, that enters the upper pole of the gland, divides into anterior and posterior branches and anastomoses with the ascending branch of the inferior thyroid artery. The upper pole is narrow, hence ligation is easy.
- The inferior thyroid artery is a branch of the thyrocervical trunk1 and enters the posterior aspect of the gland. It supplies the gland by dividing into 4 to 5 branches which enter the gland at various levels (not truly lower pole).
¹Branches of the thyrocervical trunk can be remembered as SIT-suprascapular, inferior thyroid and transverse cervical artery.
The thyroid ima artery is a branch of either the brachiocephalic trunk or the direct branch of the arch of the aorta and enters the lower part of the isthmus in about 2 to 3% of the cases.
Venous Drainage
- The superior thyroid vein drains the upper pole and enters the internal jugular vein. The vein follows the artery.
- The middle thyroid vein is single, short and wide and drains into the internal jugular vein.
- Inferior thyroid veins form a plexus which drains into an innominate vein. They do not accompany the artery.
- Kocher’s vein is rarely found (vein in between middle and inferior thyroid vein).
Nerves In Relationship With Thyroid Gland
- External laryngeal nerve: Vagus gives rise to the superior laryngeal nerve, which separates from the vagus at the skull base and divides into two branches.
- The large internal laryngeal nerve is sensory to the larynx. The small external laryngeal nerve runs close to the superior thyroid vessels and supplies cricothyroid muscle (tensor of the vocal cord) and is sensory to the upper half of the larynx.
- The superior thyroid artery and vein are like a newly married couple, they go together hand in hand, The middle thyroid vein is single, the bachelor and inferior thyroid artery and vein are a divorced couple.
- This nerve is away from the vessels near the upper pole. Hence, in thyroidectomy, the upper pedicle should be ligated as close to the thyroid as possible.
- The recurrent laryngeal nerve (RLN) is a branch of the vagus, that hooks around ligamentum arteriosum on the left and subclavian artery on the right, and runs in the tracheoesophageal groove near the posteromedial surface.
- Close to the gland, the nerve lies in between (anterior or posterior) the branches of the inferior thyroid artery.
- On the right side, it is 1 cm within the tracheo-oesophageal groove.
Lymphatic Drainage Of Thyroid
- The subcapsular lymphatic plexus drains into paratracheal nodes (delphic nodes means uncertain) and laryngeal nodes which ultimately drain into lower deep cervical nodes and mediastinal nodes.
- The chief lymph nodes are the middle and lower deep cervical lymph nodes.
Recurrent Laryngeal Nerve Anomalies
- The nerve traverses through the gland in about 5–8% of cases
- The nerve may be very closely adherent to the posteromedial aspect of the gland.
- Nerve not seen—may be far away in the trachea-oesophageal groove
- Non-recurrent-recurrent laryngeal nerve is found in about 1 in 1,000 cases.
- The nerve has a horizontal course.
- In 25% of the cases, it is within the ligament of Berry.
- Supraclavicular nodes and nodes in the posterior triangle can also be involved in malignancies of the thyroid gland, especially papillary carcinoma thyroid.
Thyroid Gland Physiology
- Tri-iodothyronine (T3) and thyroxine (T4) are the hormones secreted by the thyroid gland.
- The dietary requirement of iodine per day is 100–200 micrograms. Sources of iodine are milk, dairy products, and seafood including fish.
Steps involved in the synthesis of these hormones
- Iodide trapping from the blood into the thyroid is the first step in the formation of T3 and T4.
- Thiocyanates and Perchlorates block this step.
- Oxidation of iodide to inorganic iodine
- This step needs the enzyme peroxidase.
- Drugs which block this stage are sulfonamide, PAS (para-aminosalicylic acid),
Carbimazole, propylthiouracil, etc. (thioamides).
- Formation of iodothyronine
- Iodine + Tyrosine = MIT (Monoiodotyrosine) and Diiodotyrosine (DIT).
- This step is inhibited by thiourea group of drugs, i.e. Carbimazole.
- Coupling reactions
- Coupling of two molecules of DIT results in T4 and one molecule of DIT and MIT results in T3.
- This stage is blocked by Carbimazole.
- The hormones combine with globulin to form a colloid-thyroglobulin. They are stored in the thyroid gland and released as and when required.
- T3 is an important physiological hormone and fast fast-acting (a few hours). T4 is a slow-acting hormone and takes about 4–14 days to act.
Thyroid Function Tests
T3 and T4 estimation are the most commonly performed thyroid function tests. Other tests are not commonly done, some of them are obsolete.
- Serum T3: Tri-iodothyronine 1–2 to 3 nmol/L (0.8–2.0 ng/ml)
- Serum T4: Tetra-iodothyronine 55 to 150 nmol/L (4–12 µg/dl)
- Serum TSH: Thyroid Stimulating Hormone 0–5 IU/ml of plasma (0.3–5) Outlines the levels of T3, T4 and TSH in a few common conditions.
- Serum creatinine: In hyperthyroidism, it is increased and it is decreased in hypothyroidism.
- Serum cholesterol: It is increased in hypothyroidism and decreased in hyperthyroidism.
- Serum calcitonin: The primary role is to decrease the levels of calcium. It is increased in the Medullary Carcinoma Thyroid (MCT).
- Thyroid autoantibody levels: More than 90% of the patients with Hashimoto’s thyroiditis and 80% of patients with Grave’s disease have antibodies which are called antimicrosomal antibodies (earlier called LATS).
- The detection of these antibodies helps in the diagnosis of such cases and also to suspect these diseases before clinical manifestation.
- Thyroid scintigraphy: Uptake by both lobes.
Causes Of Hyperthyroidism
↑ Secretion of thyroid hormones
- Graves’ disease
- Plummer’s disease
- Jod-Basedow
- Amiodarone toxicity
- TSH—secreting pituitary tumours
- HCG—secreting tumours
Thyroid hormone levels without thyroid gland secretion
- Subacute thyroiditis
- Factitious hyperthyroidism
- Struma ovarii
Follicular Carcinoma
- Incidence: Constitutes 17% of cases.
- Of follicular adenomas 20% are malignant and 80% are benign.
Aetiology
Follicular carcinoma usually arises in a multinodular goitre, especially in cases of endemic goitre. It should be suspected when MNG starts growing rapidly.
Pathology
- Depending upon the property of invasion, it is classified into:
- Noninvasive which means minimal invasion.
- Invasive refers to angio-invasion and capsular invasion, necessary for the diagnosis of follicular carcinoma of the thyroid.
The tumour cells line, the blood vessels and get dislodged into the systemic circulation producing secondaries in the bones. Macroscopically most of the tumours are well-encapsulated.
Papillary Carcinoma Near Total Thyroidectomy
Reasons for Papillary Carcinoma
- Rich intrathyroidal lymphatic spread
- Multicentric origin
Clinical Presentation
- It can present as a solitary nodule. The diagnosis is considered only after a thyroid scan in which it appears cold. The peak age group is around 40 years.
- In cases of long-standing multinodular goitres, if the goitre is rapidly growing, if it is hard or if it is present with restricted mobility, follicular carcinoma can be considered.
- Metastasis in the flat bones: The only clinical situation wherein a follicular carcinoma can be considered as the diagnosis is when a patient with thyroid swelling presents with metastasis in the bone in the form of pathological fractures or pulsatile swelling.
- Commonly, secondaries develop in the flat bones like skull, ribs, sternum, vertebral column, etc., because the flat bones retain red marrow for a longer time.
The clinical features of secondary in the skull are
- They are rapidly growing
- They are warm
- Vascular and pulsatile
- Underlying bony erosion may be present
Differential Diagnosis Of Bone Secondaries Spread
- Predominantly blood spread
- In 7% of cases, lymphatic spread can occur
Papillary Carcinoma—Lymph Node Metastasis—Peculiarities
- They can be palpable even when the thyroid gland is not palpable—occult primary
- Very slow growing
- Very often, they are intracapsular
- They need not be hard, are often cystic and firm in consistency
- At operation, they are bluish in colour because of rupture of the papillae
- The presence of lymph node metastasis does not affect the prognosis
Investigations
- Routine investigations
- Ultrasound to detect a solid lesion
- FNAC of the nodule: It should be remembered that FNAC cannot differentiate a follicular adenoma from follicular carcinoma. Hence, if a patient presents with
Causes Of Secondary In The Skull
- Follicular carcinoma of the thyroid
- Carcinoma breast
- Renal cell carcinoma
- Hepatocellular carcinoma solitary nodule, the only way to get a preoperative diagnosis of follicular carcinoma is to do an incision biopsy which is NOT Acceptable.
- Alkaline phosphatase—If increased, a bone scan should be done
- Plain X-ray of the involved bone can reveal osteolytic lesions
- When the primary is not found, a bone biopsy from the secondary
- Treatment of follicular carcinoma of the thyroid
Total thyroidectomy is indicated for the following reasons:
- The secondaries do not take up the radioisotope (I131) in the presence of a primary tumour. Hence, lobectomy or hemithyroidectomy should not be done.
Malignant Tumours Wherein Open Biopsy Is Not Taken
- Thyroid
- Parotid
- Testis
- Malignant melanoma
- Patients presenting with solitary nodules should undergo hemithyroidectomy and frozen section examination. If the frozen section is positive, then a near-total thyroidectomy is done (optional detected)
- If the frozen section is negative, in high-risk patients, it is better to do near-total thyroidectomy rather than waiting for a histopathological report and subjecting the patient to second surgery.
3. After total thyroidectomy, a whole-body bone scan is done to see for metastasis in the bone.
- A single secondary can be treated by radiotherapy followed by oral radio-iodine therapy, so as to suppress the secondary. Multiple secondaries are treated by oral radio-iodine therapy.
3. In the postoperative period, patients should receive thyroxine 0.3 mg/day (reasons same as papillary carcinoma).
Hurth Le Cell Carcinoma
- Hurthle cell carcinoma is a variant of follicular carcinoma. It is more aggressive than follicular carcinoma. These tumours are defined by the presence of more than 75% of follicular cells having oncocytic features.
- Does not take up ¹³¹I.
- Secretes thyroglobulin
- Less likely to respond to 131I ablation
- Even if Hurthle cell adenoma is well encapsulated, it is potentially malignant.
- More likely to include lymph nodes.
Criteria to diagnose Hurthle Cell Carcinoma
- Capsular/vascular invasion, distant metastasis
- Higher chance of spread to lymph nodes than follicular thyroid carcinoma.
Hurth Le Cell Carcinoma Treatment
- Total thyroidectomy. In many cases of Hürthle cell carcinoma, lymph nodes are enlarged. Hence, modified radical neck dissection is done (MRND).
- TSH suppression and follow-up are regularly required. The case of FNAC proved follicular neoplasm.
Follow-up Of Patients With Papillary And Follicular Carcinoma Thyroid
- Serum thyroglobulin (Tg) levels greater than 1 to 2 ng/ml in patients receiving replacement thyroxine therapy indicate the presence of metastasis. Hence assess the serum Tg response to injected recombinant human TSH, every year.
- Ultrasonography or MRI scans of the neck for localisation of residual or recurrent tumours.
- Regular measurements of serum calcium and thyroid hormones after near-total thyroidectomy.
Graves’ Disease
- Goitre
- Ophthalmic symptoms Students can remember the symptoms of Graves’ disease as Goitre
- Irritability
- Tremors
- Restlessness
- Excitability
Past history
History of medication in the past is invariably present in toxic goitres. Previous surgery suggests a recurrent goitre.
Family history
- Pendred syndrome is a condition where congenital deafness is associated with goitre and hypothyroidism. This is due to the absence of the enzyme, peroxidase (dyshormonogenesis). Goitre can also run in families, due to dehalogenase deficiency.
- Medullary carcinoma of the thyroid can run in families.
- In endemic a
- rea’s, many family members may suffer from goitre. It is due to iodine deficiency.
- Such goitres are called endemic goitres.
General Physical Examination
- Eye signs: Graves’ disease
- Tremors of the tongue and hands: Graves’ disease
- Pulse rate: Increased in toxic goitres, decreased in myxoedema
- Pretibial myxoedema: Treated Graves’ disease
- Bony tenderness: Carcinoma thyroid
- Blood pressure changes: Secondary thyrotoxicosis
General Physical Examination On Examination Inspection
- The location of the swelling in front of the neck, extending from one sternomastoid to the other sternomastoid, vertically from the suprasternal notch to the thyroid cartilage.
- The size and shape have to be mentioned.
- Surface: Thyroid swellings can have the following types of surfaces
- Smooth —Adenoma, puberty goitre, Graves’ disease
- Irregular—Carcinoma of the thyroid
- Nodular—Multinodular goitre
- Borders are usually round
- Swelling moves up with deglutition because of the following reasons.
- The thyroid is enclosed by pre tracheal fascia which is condensed to form a ligament posteromedially called Ligament of berry.
- These are pairs of ligaments attached above to the cricoid cartilage. During deglutition, the cricoid cartilage moves upwards.
- Hence, the thyroid gland moves upwards during deglutition. (Kindly give a glass of water and check for movement with deglutition).
- Movement on protrusion of the tongue suggests a thyroglossal cyst. This test should be done when there is a nodule or a cyst in the region of the isthmus of the thyroid gland. This test has no relevance in cases of MNG or other thyroid swellings.
Swellings that Move Upwards With Deglutition
- Thyroid swellings
- Subhyoid bursitis
- Pretracheal and laryngeal lymph nodes
- Thyroglossal cyst
If there is a restriction of movement, it can be due to the following reasons:
- Malignancy with fixity to the trachea
- Retrosternal goitre
- Large goitre because of the size
- Previous surgery.
Palpation
It should be done from:
- Size, shape, surface and border should be confirmed. Local rise of temperature is a feature of toxic goitres.
- Consistency
- Soft: Graves’ disease, colloid goitre.
- Firm: Adenoma, multinodular goitre.
- Hard: Carcinoma, calcification in the MNG.
- Confirm the movement with deglutition by holding the thyroid gland.
- Intrinsic mobility of the gland is very much restricted in carcinoma because of infiltration into the trachea.
Palpation—Tests
- Local rise of temperature, size, shape, surface, borders
- Consistency, movement with deglutition
- Intrinsic mobility test
- Sternomastoid contraction test and chin test
- Position of trachea
- Palpation of lymph nodes
- Pulsations of common carotid artery
- Special tests
- Evidence of toxicity
- Stemomastoid contraction test is done where only one lobe is enlarged. In this situation, the examiner keeps the hand on the side of the chin, opposite the side of the lesion and the patient is asked to push the hand against resistance. If the gland becomes less prominent (as with thyroid swellings) it indicates the swelling is deep to the sternomastoid muscle.
- The chin test (neck fixation test) is classically done in a multinodular goitre, where both lobes are enlarged.
- The patient is asked to bend the chin downwards against resistance. This produces contraction of both sternomastoid and strap muscles, gland becomes less prominent.
- Special tests or methods of examination of the thyroid gland
- Crile’s method is indicated when there is a doubtful nodule. Keep the thumb over the suspected area of the nodule and ask the patient to swallow. The nodularity is appreciated better with this test.
- Lahey’s method of examination of the thyroid can be done from the front as well as behind. In order to palpate the right lobe, push the gland to the right side and feel the nodules in the posteromedial aspect of the gland. The lobe becomes more prominent and thus nodules are appreciated better.
- Pizzillo’s method is indicated in obese patients especially short-necked individuals: The patient is asked to clasp her hands and press against her occiput with her head extended. The thyroid gland becomes more prominent, thus, palpation becomes better.
- Kocher’s test: Gentle compression on lateral lobes producing stridor is described as positive.
- This is due to the “SCABBARD” (narrowed) trachea. Long-standing multinodular goitres causing tracheomalacia and carcinoma with infiltration into the trachea may give rise to stridor.
- Position of the trachea: In cases of solitary nodules confined to one lobe, the trachea is deviated to the opposite side. However, in cases of multinodular goitres, the trachea need not be deviated because of the symmetrical enlargement of both lobes.
- Palpation of lymph nodes in the neck If lymph nodes are significant, it indicates papillary carcinoma of the thyroid.
- Palpation of the common carotid artery: Draw a line from the mastoid process to the sternoclavicular joint. Then draw a horizontal line from the upper border of the thyroid cartilage.
- The point where these two lines meet is the site of the bifurcation of the common carotid artery. Just below this point, this artery should be palpated.
- In large multinodular goitres, the artery may be pushed laterally. Hence, pulsations are felt in the posterior triangle.
- Carcinoma of the thyroid engulfs the carotid sheath. Hence pulsations may be absent. Absent carotid artery pulsation is called “BERRY SIGN POSITIVE”. Since the lumen is not narrowed, superficial temporal artery pulsations are felt normally.
Percussion
Percussion over the sternum gives a resonant note in normal cases. In retrosternal goitres, it gives a dull note.
Auscultation
- It should be done in the upper pole for the following reasons: The superior thyroid artery is a direct branch of the external carotid artery. It is more superficial than the inferior thyroid artery.
Anatomical Features Of The Thyroid Gland
- The thyroid gland is in front of the neck
- Deep to pre-tracheal fascia
- Moves up with deglutition
- Butterfly-shaped when the whole gland is enlarged
- The presence of thrill and bruit are the features of toxic goitre.
Systemic Examination
This includes CNS and eye signs, as in Graves’ disease, examination of the skeletal system to rule out metastasis as in carcinoma of the thyroid, and examination of the cardiovascular system in cases of toxic goitre.
These have been dealt with in detail in the corresponding topics. Deep tendon reflexes also have to be elicited—there is a slow relaxation phase in hypothyroidism.
Diagnosis
It is based on the following anatomical features. It should be noted that the neural tumours arising from the vagus nerve can present in the same location but it will not move with deglutition.
Differential Diagnosis
- Simple goitre
- Toxic goitre
- Malignant goitre Solitary nodule
- Thyroiditis
- Other rare causes of thyroid enlargement.
Draw a line joining the mastoid process to the sternoclavicular joint. Draw another horizontal line at the upper border of the thyroid cartilage. The point where they meet is where the common carotid artery bifurcates. A little below one can feel common carotid artery pulsations.
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