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Tobias ECVS Exam Prep > Ch 121 - Thyroid and Parathyroid > Flashcards

Ch 121 - Thyroid and Parathyroid Flashcards

1
Q

What is unique about the thyroid blood supply of the cat?

A

In most cats, the caudal thyroid artery is absent

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2
Q

Into what structures does the lymph of the thyroids drain?

A
  • Right: Right lymphatic duct
  • Left: Tracheal duct
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3
Q

Where is ectopic thyroid tissue commonly found?

A
  • Along the trachea
  • Thoracic inlet
  • Within mediastinum
  • Thoracic portion of the descending aorta
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4
Q

How are thyroid hormones produced?

A
  • 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
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5
Q

What % of T3 and T4 circulate unbound to protein?

A

Less than 1%

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6
Q

Which thyroid hormone is more biologically active?

A

T3
Approx 40-60% of T3 is derived from monodeiodination of T4 in peripheral tissues

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7
Q

What substances regulate thyroid hormone synthesis?

A
  • Thyrotropin (TSH) from the pituitary gland
  • Thyrotropic-releasing hormone (TRH) from the hypothalamus. Modulates thyroid hormone-TSH feedback loop
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8
Q

Where is PTH made?

A

PTH is synthesised, stored and secreted by chief cells of the parathyroid gland

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9
Q

What are the main effects of PTH?

A
  • 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
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10
Q

Other than PTH, what other hormone is involved with Ca homeostasis? How does it work?

A
  • 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
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11
Q

What % of hyperthyroidism in cats is caused by carcinoma?

A

1-4%, mets in up to 74%
(Usually adenomatous hyperplasia)

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12
Q

How common is bilateral involvement of hyperthyroididm in cats?
How common is ectopic thyroid seen?

A
  • Bilateral in approx 70%
  • Ectopic hyperfunctioning tissue in 9-23%, most commonly in the chest
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13
Q

List concurrent diseases often seen with hyperthyroidism in cats

A
  • 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%
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14
Q

What imaging method is most useful for diagnosis and anatomical localisation of hyperfunctioning thyroid tissue?

A

Scintigraphy
- Technetium 99m - pertechnetate (99mTcO4)
- Trapped by thyroidal iodine-concentrating mechanisms
- Does not reflect function
- Pertechnetate normally concentrates in thryoid, salivary and gastric mucosa

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15
Q

What is the main goal of pre-op treatment of hyperthyroid cats?

A
  • Methimazole of propylthiouracil until reached a euthyroid state (6-12wks pre-op)
  • If azotaemia occurs, lifelong methimazole recommended, no irreversible treatments
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16
Q

What muscles must be seperated on the approach to the thyroid?

A
  • Sternohyoid
  • Sternothyroid
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17
Q

List the surgical options of thyroidectomy

A
  • Intracapsular (high recurrence)
  • Extracapsular (no attempt to save parathyroid)
  • Modified intracapsule
  • Modified extracapsular
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18
Q

What cand be done if a parathyroid gland is accidentally removed or its blood supply is damaged?

A

Reimplantation into a pocket of sternohyoid or sternothyroid muscle
- Function expected within 7-21d

19
Q

List potential complications of thyroidectomy

A
  • Hypoparathyroidism (less than 6%)
  • Hypothyroidism (rare, even after bilateral)
  • Recurrence - 5-11% within 2-3yr
  • Haemorrhage
  • Lar par
  • Horners syndrome
  • Dyspnoes
20
Q

How do you treat acute hypoCa?

A
  • 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

21
Q

What is prescribed for maintenance therapy of hypoCa?
For how long?

A
  • Oral Vit D (calcitriol) and calcium
  • up to 3 months. Monitored weekly and tapered as required
22
Q

What is the prognosis for thyroid carcinoma

A
  • Rare so relatively unknown
  • Marginal thyroidectomy + I131 - survival 10-41m
  • High dose I131 alone - 6/8 euthyroid with survival time 181-2381d
23
Q

List alternative treatments for hyperthyroidism in cats

A
  • 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
24
Q

What % of clinically detectable thyroid masses in dogs are malignant?
Bilateral?
Metastasise?
Functional?

A
  • 90% thyroid carcinomas
  • 25-47% bilateral
  • Up to 40% mets a diagnosis, 80% develop mets
  • 10-29% hyperthyroid
25
Q

How can iodinated contrast material used for CT scan affect further diagnostics and treatment?

A
  • Inhibitory effect on thyroid uptake of radioiodine and pertechnetate
26
Q

What blood tests are required to determine the functional status of a thyroid tumour in dogs?

A
  • Total T4
  • Free T4
  • TSH
27
Q

What intraop technique can aid in the identification of the parathyroid tissue?

A

Indocyanine green near-infrared fluorescent imaging

28
Q

How often should Ca be monitored in dogs undergoing a bilateral thryoparathyroidectomy?

A

At least daily for 3-7 days with Ca and Vit D supplementation

29
Q

List treatment options for thyroid carcinoma

A
  • Surgical excison (freely movable most amenable)
  • Radiation therapy
  • Radioactive I131
30
Q

List potential complications after thyroidectomy in dogs

A
  • Haemorrhage and anaemia
  • Hypothyroidism (40% if bilateral)
  • Laryngeal paralysis
  • Megaoesophagus
  • Hypoparathyroidism and hypocalcaemia (21/27 dogs with bilateral thyroparathyroidectomy). Many require lifelong therapy
31
Q

List prognostic factors associated with thyroid carcinoma

A
  • Mobility
  • Size
  • Stage of disease
  • Vascular invasion

Overall MST 22m

32
Q

List factors associated with thyroid carcinoma invasiveness

A
  • Diameter
  • Volume
  • Fixation
  • Ectopic location
  • Follicular cell origin

MST 3yr if freely movable vs 6-12m if more invasive

33
Q

List factors assoc with metastasis with thyroid carcinoma

A

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

34
Q

What percentage of dogs with thyroid carcinoma have ectopic disease?
What are the most common locations?

A

13% ectopic tumour
- Sublingual (may require partial hyoidectomy)
- Cranial mediastinal

Tx with surgical excision, I131 or external beam radiation therapy

35
Q

List the systemic effects of hypothyroidism which may effect anaesthesia/surgery

A

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

36
Q

What causes primary and secondary hyperparathyroidism?

A

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

37
Q

What are the pathophysiologic effects of primary hyperparathyroidism?

A

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

38
Q

List DDx for hyperCa

A
39
Q

How is primary hyperparathyroidism diagnosed?
How is this differed from renal secondary hyperparathyroidism?

A

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

40
Q

When is pre-op medical management recommended prior to Sx for hyperparathyroidism?
What does this encompass?

A

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

41
Q

What are the surgical options for external parathyroidectomy and internal parathyroidectomy?

A

External:
- Parathyroidectomy
- Partial thryoidectomy

Internal:
- Partial thyroidectomy
- If carcinoma, complete thryoparathyroidectomy

42
Q

List methods of intra-op detection of abnormal parathyroid glands

A

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

43
Q

List potential complications after parathyroidectomy

A
  • 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)

44
Q

List alternative therapies for primary hyperparathyroidism

A
  • Ultrasound guided ethanol ablation
  • Ultrasound-guided heat ablation

Inconsistent results

Tobias ECVS Exam Prep (110 decks)

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