Thyroid and Parathyroid Glands Flashcards
First embryological stage of thyroid development
Epithelial proliferation at the site of the foramen caecum at the base of the tongue in week 4
When does the thyroid gland reach its final position during development
24 weeks
How is the thyroid connected to the tongue during development
Thyroglossal duct
What is the fate of the thyroglossal duct
Initially solidifies before finally disappearing
Describe the origin and location of thyroglossal cysts
- Cystic remnants of the thyroglossal duct
- Can be found along its path of descent
List the potential sites of accessory thyroid tissue
- Tongue
- Near hyoid bone
- Deep to SCM
- Superior mediastinum
Describe the principles of thyroglossal cyst removal
Removal of the cyst, track, and origin is mandatory (as the track can loop behind the hyoid, the central portion of the hyoid is usually excised with the tract)
When do thyroglossal fistulae arise
When a thyroglossal cyst is incised and drained in the mistaken belief that it is a simple abscess
What hormones are produced by the thyroid gland
- Thyroxine (T4)
- Triiodothyronine (T3)
- Calcitonin
Where is calcitonin secreted from
Parafollicular C cells of the thyroid
Which thyroid cancer produces calcitonin
Medullary thyroid cancer (cancer of the parafollicular C cells)
Which of T3 and T4 is faster acting and why
T3 - as T4 has greater affinity for binding proteins
T3 half life
1 day
TSH half life
5 minutes
List the 8 stages of thyroid hormone synthesis
- Trapping
- Oxidation
- Secretion into Colloid
- Binding
- Pinocytosis
- Proteolysis
- Secretion into plasma
- Transport
What catalyses the conversion of iodide to iodine
Thyroid peroxidase
When is TBG raised
- Pregnancy
- Oestrogen therapy
When is TBG reduced
- Liver disease
- Nephritis syndrome
- SLE
What is the clinical use of thyroglobulin
Monitoring of follicular and papillary thyroid cancers
Describe the 3 components of the thyroid gland
- Isthmus (overlying 2nd and 3rd tracheal rings)
- Lateral lobes (extend to 6th tracheal ring)
- Inconstant pyramidal lobe extending up from the isthmus
What connects the thyroid to the cricoid cartilage and trachea
Berry’s ligament
Anterior relations of the thyroid
- Strap muscles
- SCM
- Enclosed in pretracheal fascia
Posterior relations of the thyroid
- Larynx
- Trachea
- Pharynx
- Oesophagus
Lateral relations of the thyroid
Carotid sheath
Outline the arterial supply of the thyroid gland
- Superior thyroid artery from ECA passes to upper pole (closely related to external branch of superior laryngeal nerve)
- Inferior thyroid artery from thyrocervical trunk is related to the recurrent laryngeal nerve close to the gland
- Thyroidea ima (inconstant)
Outline the venous drainage of the thyroid gland
- Superior thyroid vein drains upper pole to IJV
- Middle thyroid vein drains to IJV
- Inferior thyroid vein drains lower pole to brachiocephalic vein
Thyroidectomy incision
Transverse incision two finger’s width above the sternal notch
Structures encountered when approaching the thyroid gland via transverse incision
- Platysma
- Investing fascia
- Strap muscles (divided in upper half)
- Pretracheal fascia
Where should the superior thyroid artery be divided when performing thyroidectomy and why
Close to the gland to avoid damage to the superior laryngeal nerve
Where should the inferior thyroid artery be divided when performing thyroidectomy and why
Far from the gland to avoid damage to the recurrent laryngeal nerve
Describe a diffuse non-toxic goitre
Diffuse enlargement involving the whole gland without producing nodularity and is not associated with hypo- or hyperthyroidism
Causes of non-toxic simple goitre
- Physiological due to increased demand for thyroid hormones e.g. pregnancy
- Dietary iodine deficiency
- Treated Grave’s
- Lymphoma
- Anaplastic carcinoma
- Autoimmune thyroiditis
- De Quervain’s thyroiditis
Treatment of non-toxic simple goitre
- Small = conservative
- Large = hemi- or total thyroidectomy
Pathophysiology of non-toxic simple goitre
Hypertrophy AND hyperplasia secondary to a relative reduction in output of T3 and T4. Causes rise in TSH which causes gland enlargement.
What is Plummer’s disease
Autonomous hyperfunctioning (hyperthyroid) nodule in a multinodular goitre
Treatment of multinodular goitre
- Medical - aim to suppress TSH to zero with thyroxine
2. Surgical - hemi- or total thyroidectomy
Indications for multinodular goitre surgery
- Local symptoms e.g. dysphagia
- Enlarging dominant nodule
- RLN palsy
- Cosmesis
- Hyperthyroidism
Cause of true solitary nodules
- 80% are adenomas
- 10% are cancer (papillary)
- 10% are cysts, fibrosis or thyroiditis
GOLD standard investigation of thyroid nodules
FNAC
Action to be taken for FNAC Thy 3
- Follicular lesion (35% risk of malignancy)
- Diagnostic lobectomy should be performed
- If cancerous then thyroidectomy with level 6 dissection
Action to be taken for FNAC Thy 4
- Suspicious for papillary malignancy
- Thyroidectomy with level 6 dissection
What does a total lobectomy involve
Removal of a single lobe and the isthmus
Most common type of thyroid cancer
Papillary - 70%
Most common site of spread in papillary thyroid cancer
Lymphatic spread to cervical nodes
Histological features of papillary thyroid cancer
- No capsule
- May be multifocal
- ‘Orphan Annie’ nuclei
- Psammoma calcification is diagnostic
What are Psammoma bodies
Clusters of calcification, diagnostic of papillary thyroid cancer
Treatment of papillary thyroid cancers >1cm (controversial as most treated as if >1cm)
- Total thyroidectomy with neck dissection
- Radioiodine ablation
- Lifelong TSH suppression with T4
- Annual thyroglobulin
- Lifelong follow-up
How are papillary thyroid metastases detected
Whole body scintography with iodine-131
Most common site of spread of Follicular thyroid cancer
- Bone
- Lungs
Histological features of Follicular thyroid cancer
- Capsulated (but if breached = carcinoma)
- Cannot be distinguished as malignant on FNAC alone
How is a diagnosis of follicular thyroid cancer made
- Cannot be diagnosed solely on FNAC
- Thyroid lobectomy
- If capsular transgression or vascular invasion then the other lobe is removed also
Sole histological difference between follicular carcinoma and adenoma
Capsule is breached in carcinoma
Describe medullary thyroid carcinomas
Tumour of calcitonin-secreting parafollicular C-cells
Biochemical marker for medullary thyroid cancer
Calcitonin
Treatment of medullary thyroid cancer
- Total thyroidectomy and neck dissection
- T4 given but doesnt need to suppress TSH
- Monitor calcitonin (rising indicates mets)
Genetic associations of medullary thyroid cancer
- 20% genetic
- MEN 2A
Most likely cause of advanced thyroid cancer
Anaplastic carcinoma
Age distribution of anaplastic thyroid cancer
- Older patients
- 60-70
Treatment of anaplastic thyroid cancer
- Debulking surgery
- Palliative external-beam radiotherapy
- 90% die within 1 year
What must be excluded when suspecting anaplastic thyroid cancer
Lymphoma - can be treated!
Associations of thyroid lymphoma
Hashimoto’s thyroiditis
Treatment of thyroid lymphoma
- Chemotherapy
- Occasionally radiotherapy
- Surgery if localised or persistent disease
Investigations for thyroid lymphoma
- Core or open biopsy
2. Staging CT
Describe routine neck dissection in thyroid malignancy
- Thyroid glands drain to lymph nodes in level 6 in the middle of the neck
- Region extends from hyoid to suprasternal notch and laterally to IJV and RLN
- SCM and IJV preserved
Which thyroid cancer patients may receive radioactive iodine
- Papillary cancers >1cm and/or node positive
- Follicular cancers
What is performed to make radioactive iodine treatment more effective
Patients are rendered hypothyroid prior to treatment to increase iodine uptake
Describe thyroid adenomas
- Benign lesion (mostly follicular adenomas)
- Discrete lesion with glandular or acinar pattern
- Encapsulated
- Can cause hyperthyroidism
How are thyroid adenomas diagnosed
Thyroid lobectomy (cannot be differentiated from follicular carcinoma on FNAC)
Cause of acute thyroiditis
Bacterial - usually streptococci
Features of acute thyroiditis
- Pain, tenderness and erythema over the thyroid
- Fever
- Raised ESR and WBC count
- TFT normal
- FNA for MC&S
Treatment of acute thyroiditis
Antibiotics and analgesia (occasionally steroids)
Describe subacute (De Quervain) thyroiditis
- Granulomatous thyroiditis
- Caused by viral infection
Cause of De Quervain thyroiditis
Viral:
- Mumps
- EBV
- Measles
Clinical features of De Quervain thyroiditis
- Painful goitre
- Period of hyperthyroid followed by hypothyroid then euthyroid
- ESR and WBC count raised
Treatment of De Quervain Thyroiditis
- Aspirin
- Prednisolone
- 6-8 weeks
Antibodies detected in Hashimoto’s thyroiditis
- TPO antibodies
- Thyroglobulin antibodies
Cause of Hashimoto’s thyroiditis
- Defect in T-cell function
- Allows T-helper cells to be sensitised to thyroid antigens
- B-cells are stimulated to produce anti-thyroid antibodies
Describe Riedel’s thyroiditis
- Idiopathic fibrosing condition
- 30-40% end up hypothyroid
- Palliative treatment
MOA and type of Radio-iodine used in the treatment of Grave’s disease
- Iodine-131
- Single oral dose
- Causes direct damage to the replicative mechanism of the thyroid follicular cells
Pre-operative requirements in thyroidectomy
All hyperthyroid patients should be rendered euthyroid:
- Carbimazole and propylthiouracil
- Beta-blockers
- Iodine
Symptoms of unilateral RLN palsy
- Weak breathy voice
- Bovine cough
Treatment of laryngeal oedema following thyroidectomy
48 hours of ventilation
Describe tracheomalacia
- Rare complication following removal of large goitre
- Trachea collapses after loss of its support
- Require tracheostomy
Why is hypocalcaemic tetany a complication of thyroid surgery
Accidental removal of the parathyroid glands causes drastic reduction in PTH
Symptoms of external laryngeal nerve injury
Weak voice when trying to sing or shout
Consequence of external laryngeal nerve injury
Cricothyroid muscle paralysis
Origin of the superior parathyroid glands
Dorsal endoderm of the 4th branchial pouch
Origin of the inferior parathyroid glands
3rd branchial pouch (along with the thymus)
Which parathyroid glands are more likely to migrate
- Inferior
- Descend with thymus
Ectopic positions of the parathyroid glands
Superior glands:
- 90% constant
- Behind oesophagus
- Carotid sheath
Inferior glands:
- Along inferior thyroid veins
- In front of trachea
- Superior mediastinum with thymus
Parathyroid arterial supply
Inferior thyroid artery
Venous drainage of parathyroids
Thyroid venous plexus which drains into brachiocephalic vein
Histology of parathyroid cells
- Mainly chief cells
- Some oxyphil cells
- Some water-clear cells
Where is PTH released from
Chief cells of the parathyroid glands
PTH half-life
2-5 minutes
Where does PTH elicit its effect and how
- Bone
- Gut
- Kidney
Via cell surface receptors, and increases cAMP production
% of total body calcium that is free in the extracellular fluid
1%
Most common presentation of parathyroid pathology
Hypercalcaemia
Most common causes of primary hyperparathyroidism
- Single adenoma (85%)
2. Parathyroid hyperplasia (12%)
Primary hyperparathyroidism biochemical presentation
- Hypercalcaemia
- Normal or high PTH
- Hypophosphataemia
- Hypochloraemia
- Mild acidosis
- Normal vitamin D level
Describe the structure of parathyroid adenomas
- Consist of chief cells
- Monoclonal
- Rim of compressed normal parathyroid tissue at the periphery
- Capsule with loss of fat and stroma within the adenoma
Pathophysiology of secondary hyperparathyroidism
Excessive PTH production by the parathyroid glands in response to low calcium (e.g. renal failure of malabsorption)
Biochemical presentation of secondary hyperparathyroidism
- Normal calcium
- High phosphate
- Very high PTH
How is the incidence of secondary hyperparathyroidism prevented against in renal failure
- Low phosphate diet
- Activated vitamin D
Inheritance pattern of familial hypocalciuric hypercalcaemia
Autosomal dominant
Symptoms of hypercalcaemic crisis
- Drowsiness/confusion
- LOC/coma
- Dehydration
- Weakness
- Vomiting
- Renal failure
Treatment of primary hyperparathyroidism
Surgical intervention is the only cure (to prevent end-organ damage)
When is bilateral exploration of the neck in hyperparathyroidism indicated to look for multiple adenomas
- MEN1 or 2A
- Known bilateral disease
- If hyperplasia is seen in two glands on the same side of the neck
- If no abnormality is found on unilateral exploration
How is excised tissue confirmed to be parathyroid tissue
Immediate frozen section
Post-op medication to be given in parathyroid surgery
1 alpha-calcidol supplements
List the complications of parathyroidectomy
- As for thyroid surgery
- Damage to RLN less so than in thyroid surgery
- Hypoparathyroidism and hypocalcaemia
- Bleeding/haematoma
- Inability to identify all four glands
Indications for surgery in primary hyperparathyroidism
- Elevated serum calcium >1mg/dl above normal
- Hypercalciuria >400mg/day
- Creatinine clearance <30% of normal
- Episode of life-threatening hypercalcaemia
- Nephrolithiasis
- Age <50
- Neuromuscular symptoms
- Reduction in BMD >2.5 SD below peak bone mass
Procedure for single parathyroid adenoma
Remove the affected gland and leave the 3 remaining glands
Procedure for multiple parathyroid adenomas
Remove all affected glands
Procedure for parathyroid hyperplasia
- Excise 3.5 glands
- Mark the remaining gland or autotransplant into brachioradialis
Procedure for parathyroid carcinoma
En-bloc dissection with thyroid lobectomy and lymph nodes
Procedure for hyperparathyroidism in MEN
- Total parathyroidectomy
- Autotransplantation
- Thymectomy and exploration of the carotid sheath
List the causes of hypoparathyroidism
- Iatrogenic
- Autoimmune
- Pseudohypoparathyroidism (decreased sensitivity to PTH)
- Congenital e.g. DiGeorge syndrome
Non-parathyroid-related causes of hypocalcaemia
- Acute pancreatitis
- Small bowel disease e.g. CD or large resection
- Post-vagotomy
- Massive blood transfusion
- Renal failure
List the features of MEN 1
- Parathyroid hyperplasia
- Pancreatic and duodenal endocrine tumours
- Pituitary adenoma
- Thyroid adenoma
- Adrenal adenoma or carcinoma
- Carcinoid
- Lipoma
List the features of MEN 2A
- Parathyroid hyperplasia
- Phaeochromocytoma
- Medullary carcinoma of the thyroid
List the features of MEN 2B
- Phaeochromocytoma
- Medullary carcinoma of the thyroid
- Mucosal and ganglioneuromas
- Marfanoid appearance
Result and management of the RET mutation
- Causes MEN 2 and can be screened for
- All should have thyroidectomy
What can be used to stain the parathyroid glands intraoperatively
Methylene blue
