Hyperthyroidism

Question 1

Thyroid gland physiology

Answer 1

Follicular cells (thyrocytes)
Lumen filled with colloid
Parafollicular (C) cells (secrete calcitonin)

Question 2

Production of the thyroid hormone

Answer 2

Na+/I- symporter transports Na ions across membrane with iodide ion (iodide trapping) → TSH released form pituitary and binds to TSH receptor stimulating the endocytosis of the colloid → Endocytosed vesicles fused with lysosomes of the follicular cell → thyroid hormones released

Question 3

Thyroid hormones released

Answer 3

r-T3 (biologically inactive)
Triiodothyronine/ T3 (active)
Thyroxine (T4)

Question 4

Function of thyroid glands

Answer 4

Catabolic on muscle and adipose tissue, stimulate erythropoiesis
Needed for normal skeletal/ neurologic maturation
Regulate cholesterol synthesis/ degradation

Question 5

Hyperthyroidism in cats

Answer 5

Most common endocrine dz of cats caused by neoplasia and hyperplasia (most benign)

Question 6

Hyperplasia causing hyperthyroidism in cats

Answer 6

Adenomatous hyperplasia on one or both lobes
Nodules formed from <1 mm to >3mm

Question 7

Hyperthyroidism in dogs

Answer 7

Rare
Thyroid carcinoma (10-20% functional)

Question 8

CS of hyperthyroidism

Answer 8

WL with polyphagia
Hyperactivity and ↑ vocalization
PU/PD, V/D
Sudden blindness, heart murmur

Question 9

Signalment of hyperthyroidism

Answer 9

Middle-aged to old cats (4-22y)

Question 10

PE for a cat with hyperthyroidism

Answer 10

Palpate for thyroid nodule/ slip (not all will have)
Poor BCS, dull hair coat, dehydration, hyperactivity, tachycardia

Question 11

Clin path associated with hyperthyroidism

Answer 11

↑ ALT, ALKP (return to normal @ euthyroid)

Question 12

Dx hyperthyroidism

Answer 12

Palpation of enlarged node
Free T4 equilibirum dialysis: ↑ T4

Question 13

Subclinical hyperthyroidism

Answer 13

Thyroid slip with no CS or consistent lab abnormalities (not relying on TSH mechanism)
Low TSH, normal T4 and fT4

Question 14

Hyperthyroidism and Chr. Kidney Dz

Answer 14

Concurrent renal dysfunction common in untx cats
↓ GFR, ↑ BUN and creatinine, azotemia

Question 15

Contribution of hyperthyroidism to CKD

Answer 15

Untx hyperthyroid cats develop proteinuria
High levels of retinol binding protein (tubular dysfunction and damage)
High urinary N-acetyl-beta-D-glucosaminidase

Question 16

Other dx for hyperthyroid

Answer 16

BP (hypertension)
ECG: Tall R waves
Thoracic rads: Cardiomegaly, pleural effusion
Echo: HCM
Pertechnetate scan (nuclear medicine scan)

Question 17

Tx the cardiovascular effects of hyperthyroid

Answer 17

Beta blockers (atenolol)
if tachycardic/ hypertensive

Question 18

Tx for hyperthyroidism

Answer 18

Methimazole: blocks thyroid hormone synthesis by stopping thyroid peroxidase
(reversible effects)

Question 19

Adverse effects of methimazole

Answer 19

GI most common
Blood dyscrasias (neutropenia and thrombocytopenia)
Facial excoriation
Hepatotoxicity (hep necrosis and degeneration)
Renal decompensation
Coagulation abnormalities
Acquired MG

Question 20

Monitoring methimazole therapy

Answer 20

CBC, Chem, UA and T4 @ 2-3w and 4-6w
T4 1.5-3.0 ug/dL

Question 21

Nutritional management of hyperthyroid

Answer 21

Dietary Hill’s y/d
A reduction of iodine as a substrate for thyroid hormone production

Question 22

Thyroidectomy

Answer 22

Unilateral or bilateral (usually both taken)
Extracapsular: high risk, cautery cr. thyroid artery
Intracapsular: recurrence, remove some capsule

Question 23

Complication of the thyroidectomy?

Answer 23

Secondary cardiomyopathy
Recurrent or incomplete removal
Hypoparathyroidism → post-op hypocalcemia

Question 24

Radioiodine therapy

Answer 24

Tx of choice if bilateral, ectopic thyroid carcinomas
Initial medical therapy trial to ensure renal stability
Isolated for 7-10d after tx

Question 25

I131 mechanism of action

Answer 25

Concentrated in thyroid gland where it will irradiate and destroy the hyperfunctioning tissue
Emits B-particles (travels short distances) and Y-radiation

Question 26

I131 adverse effects

Answer 26

Rare transient dysphagia, fever and voice change
Worsening renal dz
Hypothyroidism (with azotemia)

Question 27

I131 monitoring

Answer 27

T4 @ 4w and 3m post-tx

Question 28

Monitoring hyperthyroid prior to I131 tx

Answer 28

Thoracic rads within 30d of tx
CBC, Chemistry screen, diagnostic T4 by outside lab
UA with UPC

Question 29

After I131 tx

Answer 29

Patients released to owners according to strict federal regulations
Excrete small amounts of radioiodine release