Hyperthyroidism Flashcards
Thyroid gland physiology
Follicular cells (thyrocytes)
Lumen filled with colloid
Parafollicular (C) cells (secrete calcitonin)
Production of the thyroid hormone
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
Thyroid hormones released
r-T3 (biologically inactive)
Triiodothyronine/ T3 (active)
Thyroxine (T4)
Function of thyroid glands
Catabolic on muscle and adipose tissue, stimulate erythropoiesis
Needed for normal skeletal/ neurologic maturation
Regulate cholesterol synthesis/ degradation
Hyperthyroidism in cats
Most common endocrine dz of cats caused by neoplasia and hyperplasia (most benign)
Hyperplasia causing hyperthyroidism in cats
Adenomatous hyperplasia on one or both lobes
Nodules formed from <1 mm to >3mm
Hyperthyroidism in dogs
Rare
Thyroid carcinoma (10-20% functional)
CS of hyperthyroidism
WL with polyphagia
Hyperactivity and ↑ vocalization
PU/PD, V/D
Sudden blindness, heart murmur
Signalment of hyperthyroidism
Middle-aged to old cats (4-22y)
PE for a cat with hyperthyroidism
Palpate for thyroid nodule/ slip (not all will have)
Poor BCS, dull hair coat, dehydration, hyperactivity, tachycardia
Clin path associated with hyperthyroidism
↑ ALT, ALKP (return to normal @ euthyroid)
Dx hyperthyroidism
Palpation of enlarged node
Free T4 equilibirum dialysis: ↑ T4
Subclinical hyperthyroidism
Thyroid slip with no CS or consistent lab abnormalities (not relying on TSH mechanism)
Low TSH, normal T4 and fT4
Hyperthyroidism and Chr. Kidney Dz
Concurrent renal dysfunction common in untx cats
↓ GFR, ↑ BUN and creatinine, azotemia
Contribution of hyperthyroidism to CKD
Untx hyperthyroid cats develop proteinuria
High levels of retinol binding protein (tubular dysfunction and damage)
High urinary N-acetyl-beta-D-glucosaminidase
Other dx for hyperthyroid
BP (hypertension)
ECG: Tall R waves
Thoracic rads: Cardiomegaly, pleural effusion
Echo: HCM
Pertechnetate scan (nuclear medicine scan)
Tx the cardiovascular effects of hyperthyroid
Beta blockers (atenolol)
if tachycardic/ hypertensive
Tx for hyperthyroidism
Methimazole: blocks thyroid hormone synthesis by stopping thyroid peroxidase
(reversible effects)
Adverse effects of methimazole
GI most common
Blood dyscrasias (neutropenia and thrombocytopenia)
Facial excoriation
Hepatotoxicity (hep necrosis and degeneration)
Renal decompensation
Coagulation abnormalities
Acquired MG
Monitoring methimazole therapy
CBC, Chem, UA and T4 @ 2-3w and 4-6w
T4 1.5-3.0 ug/dL
Nutritional management of hyperthyroid
Dietary Hill’s y/d
A reduction of iodine as a substrate for thyroid hormone production
Thyroidectomy
Unilateral or bilateral (usually both taken)
Extracapsular: high risk, cautery cr. thyroid artery
Intracapsular: recurrence, remove some capsule
Complication of the thyroidectomy?
Secondary cardiomyopathy
Recurrent or incomplete removal
Hypoparathyroidism → post-op hypocalcemia
Radioiodine therapy
Tx of choice if bilateral, ectopic thyroid carcinomas
Initial medical therapy trial to ensure renal stability
Isolated for 7-10d after tx
I131 mechanism of action
Concentrated in thyroid gland where it will irradiate and destroy the hyperfunctioning tissue
Emits B-particles (travels short distances) and Y-radiation
I131 adverse effects
Rare transient dysphagia, fever and voice change
Worsening renal dz
Hypothyroidism (with azotemia)
I131 monitoring
T4 @ 4w and 3m post-tx
Monitoring hyperthyroid prior to I131 tx
Thoracic rads within 30d of tx
CBC, Chemistry screen, diagnostic T4 by outside lab
UA with UPC
After I131 tx
Patients released to owners according to strict federal regulations
Excrete small amounts of radioiodine release
