Ch 121 - Thyroid and Parathyroid Flashcards
What is unique about the thyroid blood supply of the cat?
In most cats, the caudal thyroid artery is absent
Into what structures does the lymph of the thyroids drain?
- Right: Right lymphatic duct
- Left: Tracheal duct
Where is ectopic thyroid tissue commonly found?
- Along the trachea
- Thoracic inlet
- Within mediastinum
- Thoracic portion of the descending aorta
How are thyroid hormones produced?
- Thyroglobin produced within the thyroid and stored within the thyroid follicle (Sufficient iodine is necessary for production of thyroglobin)
- Thyroglobin moves into follicular cells and is hydrolysed into thyroxine (T4) and triiodothyronine (T3) which are released into the blood
What % of T3 and T4 circulate unbound to protein?
Less than 1%
Which thyroid hormone is more biologically active?
T3
Approx 40-60% of T3 is derived from monodeiodination of T4 in peripheral tissues
What substances regulate thyroid hormone synthesis?
- Thyrotropin (TSH) from the pituitary gland
- Thyrotropic-releasing hormone (TRH) from the hypothalamus. Modulates thyroid hormone-TSH feedback loop
Where is PTH made?
PTH is synthesised, stored and secreted by chief cells of the parathyroid gland
What are the main effects of PTH?
- Increase Ca conc
- Decrease phosphorus conc
- Bone: Ca and phosphate reabsorption
- Kidneys: Rapid decrease in excretion of Ca and increase in excretion of phosphorus. Increased formation of 1,25-dihydroxycholecalciferol (calcitriol) from Vit D
- Intestines: Calcitriol increases absorption of Ca and phosphorus
Other than PTH, what other hormone is involved with Ca homeostasis? How does it work?
- Calcitonin (produced by thyroid glanf parafollicular cells, aka C-cells)
- Prevent postprandial hyperCa by decreasing bone resorption but has no effect at level of kidneys or intestines
What % of hyperthyroidism in cats is caused by carcinoma?
1-4%, mets in up to 74%
(Usually adenomatous hyperplasia)
How common is bilateral involvement of hyperthyroididm in cats?
How common is ectopic thyroid seen?
- Bilateral in approx 70%
- Ectopic hyperfunctioning tissue in 9-23%, most commonly in the chest
List concurrent diseases often seen with hyperthyroidism in cats
- Cardiac disease (Tachycardia, murmurs, gallop, HCM, sometimes hypertension)
- Renal insufficency in up to 40%. Trial course of methimazole recommended prior to any irreversible treatments
- Hypokalaemis in 32%
What imaging method is most useful for diagnosis and anatomical localisation of hyperfunctioning thyroid tissue?
Scintigraphy
- Technetium 99m - pertechnetate (99mTcO4)
- Trapped by thyroidal iodine-concentrating mechanisms
- Does not reflect function
- Pertechnetate normally concentrates in thryoid, salivary and gastric mucosa
What is the main goal of pre-op treatment of hyperthyroid cats?
- Methimazole of propylthiouracil until reached a euthyroid state (6-12wks pre-op)
- If azotaemia occurs, lifelong methimazole recommended, no irreversible treatments
What muscles must be seperated on the approach to the thyroid?
- Sternohyoid
- Sternothyroid
List the surgical options of thyroidectomy
- Intracapsular (high recurrence)
- Extracapsular (no attempt to save parathyroid)
- Modified intracapsule
- Modified extracapsular
What cand be done if a parathyroid gland is accidentally removed or its blood supply is damaged?
Reimplantation into a pocket of sternohyoid or sternothyroid muscle
- Function expected within 7-21d
List potential complications of thyroidectomy
- Hypoparathyroidism (less than 6%)
- Hypothyroidism (rare, even after bilateral)
- Recurrence - 5-11% within 2-3yr
- Haemorrhage
- Lar par
- Horners syndrome
- Dyspnoes
How do you treat acute hypoCa?
- 0.5-1.5ml/kg 10% Ca gluconate slowly IV, with ECG monitoring
- Followed by CRI of Ca gluconate at 5-15mg/kg/hr IV
- Alternatively 10ml 10% Ca gluconate added to 250ml bag and administered at 60ml/kg over 24hr
- Do not add Ca to LRS as it precipitates
- Do not used Ca carbonate IV or SQ
What is prescribed for maintenance therapy of hypoCa?
For how long?
- Oral Vit D (calcitriol) and calcium
- up to 3 months. Monitored weekly and tapered as required
What is the prognosis for thyroid carcinoma
- Rare so relatively unknown
- Marginal thyroidectomy + I131 - survival 10-41m
- High dose I131 alone - 6/8 euthyroid with survival time 181-2381d
List alternative treatments for hyperthyroidism in cats
- Methimazole or carbimazole
- Iodine-restricted diets (y/d) - 75-90% become euthyroid
- Radioactive iodine (I131) - Tx of choice. Single dose results in cure in most and ectopic tissue is also treated
What % of clinically detectable thyroid masses in dogs are malignant?
Bilateral?
Metastasise?
Functional?
- 90% thyroid carcinomas
- 25-47% bilateral
- Up to 40% mets a diagnosis, 80% develop mets
- 10-29% hyperthyroid
How can iodinated contrast material used for CT scan affect further diagnostics and treatment?
- Inhibitory effect on thyroid uptake of radioiodine and pertechnetate
What blood tests are required to determine the functional status of a thyroid tumour in dogs?
- Total T4
- Free T4
- TSH
What intraop technique can aid in the identification of the parathyroid tissue?
Indocyanine green near-infrared fluorescent imaging
How often should Ca be monitored in dogs undergoing a bilateral thryoparathyroidectomy?
At least daily for 3-7 days with Ca and Vit D supplementation
List treatment options for thyroid carcinoma
- Surgical excison (freely movable most amenable)
- Radiation therapy
- Radioactive I131
List potential complications after thyroidectomy in dogs
- Haemorrhage and anaemia
- Hypothyroidism (40% if bilateral)
- Laryngeal paralysis
- Megaoesophagus
- Hypoparathyroidism and hypocalcaemia (21/27 dogs with bilateral thyroparathyroidectomy). Many require lifelong therapy
List prognostic factors associated with thyroid carcinoma
- Mobility
- Size
- Stage of disease
- Vascular invasion
Overall MST 22m
List factors associated with thyroid carcinoma invasiveness
- Diameter
- Volume
- Fixation
- Ectopic location
- Follicular cell origin
MST 3yr if freely movable vs 6-12m if more invasive
List factors assoc with metastasis with thyroid carcinoma
Bilateral disease and tumour size
- 14% if less than 23cm^3
- 74% for 23-100cm^3
- 100% over 100cm^3
Bilateral tumours are 16x more likely to met
What percentage of dogs with thyroid carcinoma have ectopic disease?
What are the most common locations?
13% ectopic tumour
- Sublingual (may require partial hyoidectomy)
- Cranial mediastinal
Tx with surgical excision, I131 or external beam radiation therapy
List the systemic effects of hypothyroidism which may effect anaesthesia/surgery
Cardiovascular
- Decreased contractility
- Increased vascular resistance
- Decreased vascular volume
- Artherosclerosis
Suppression of humoral immune response, impariment of T-cell function and reduction in number of circulating lymphocytes
Effect on wound healing varies by species but may disturb fibroblast deposition of collagen, keratinocyte proliferation
What causes primary and secondary hyperparathyroidism?
Primary - Excessive production and secretion of PTH by abnormal, autonomously functioning parathyroid chief cells. Keeshond predisposed. Maye also Siamese
Secondary:
- Renal secondary hyperparathyroidism
- Nutritional secondary hyperparathyroidism - imbalances in phosphorus, Vit D or Ca
What are the pathophysiologic effects of primary hyperparathyroidism?
Skeletal effects
- Fibrous osteodystrophy
- Pathologic fractures
Renal effects:
- Renal tubular mechanisms become overwhelmed and excessive Ca excretion results
- Urolithiasis, UTI, PU/PD, diabetes insipidus
- If Ca x P product is greater that 60-80 mg/dL, soft tissues calcify, causing renal dysfunction
- Renal vasoconstriction
About 4% of dogs with primary hyperparathyroidism will have renal damage
List DDx for hyperCa
How is primary hyperparathyroidism diagnosed?
How is this differed from renal secondary hyperparathyroidism?
Inappropriate PTH (normal or high) on the face of increased ionised Ca. Low PTH-rp
Differes from CKD (renal secondary hyperparathyroidism) as hyperparathyroidism will have decreased phosphate (as apposed to elevated) and are not azotaemic. PTH-rp may also be elevated in dogs with CKD without malignancy
When is pre-op medical management recommended prior to Sx for hyperparathyroidism?
What does this encompass?
Medicam management if concurrent renal failure or Ca-phosphorus ratio greater than 60-70
- Saline diuresis 120-180ml/kg/d
- Furosemide
- K supplementation PRN
- If above fails, glucocorticoids can promote calciuresis
- If all above fails, bisphosphonates or calcitonin
What are the surgical options for external parathyroidectomy and internal parathyroidectomy?
External:
- Parathyroidectomy
- Partial thryoidectomy
Internal:
- Partial thyroidectomy
- If carcinoma, complete thryoparathyroidectomy
List methods of intra-op detection of abnormal parathyroid glands
Intra-op measurement of PTH with rapid chemiluminescent assay
- Decrease by less than 50% baseline after 10 mins indicates autonomously function tissue remaining
IV methylene blue
- Dose required has too high a risk of toxicity (Heinz body anaemia and acute renal failure)
- NOT recommended
Indocyanine green near-infrared imaging
- promising experimentally
List potential complications after parathyroidectomy
- HypoCa - 35-70%, 25% of which develop clinical signs between 12hr-20d
- Recurrence - 8% within months to years. Second surgery usually highly successful
Overall prognosis is excellent, even with parathyroid carcinoma (Hyper Ca resolved in 44/47 with median follow up of 561d)
List alternative therapies for primary hyperparathyroidism
- Ultrasound guided ethanol ablation
- Ultrasound-guided heat ablation
Inconsistent results
