Hyper T4

Question 1

What is hyperT4

Answer 1

• Most commonly diagnosed endocrinopathy in cats
  o Functional thyroid adenomatous hyperplasia
  o Rare thyroid carcinoma (more frequent in dogs)
• Multisystemic disorder
• incr circulating [thyroxine, T4] and [triiodothyronine, T3]
• Can induce reversible CM => consider in any cat w CM >6y/o

Question 2

Systemic effects of T4

Answer 2

• incr metabolic rate and tissue O2 consumption
• incr SVR : T4 => vasodilation => incr tissue perfusion
  o This mechanism is outweighed by incr blood volume and contractility
• incr plasma volume + BP
  o incr ANP
  o incr ET-1
  o incr AngII + aldosterone
• incr symp system activity
  o decr A R in several organs
  o B R: T4 will act exclusively subtype 1
   incr # of B R
• Cardiac remodelling

Question 3

how T4 stimulates cardiac hypertrophy

Answer 3

 Direct action on cardiomyocytes
* Myosin isoform synthesis: incr contraction velocity
o Influence expression of myosin heavy chain genes
* incr Na+/K+-ATPase, Ca2+-ATPase, Ca2+/Na+-ATPase activity
o incr reuptake of Ca2+ durng diastole
* Changes in myocardial protein synthesis/degradation

 incr symp system activity
* incr responsiveness to symp system activity

o incr CO
o incr cardiac filling pressures
o incr contractility
o incr HR: affect K+, Na+ et Ca2+ channels => decr action potential duration => incr SA node PM activity
 decr refractory period => predispose to arrhythmias
* Afib, BB, prolonged QT

Question 4

how does CHF develop

Answer 4

depend on
o Chronicity
o Rate of development of thyrotoxic state
o Presence/severity of underlying cardiac disease

Question 5

Echo changes w/ hyperT4

Answer 5

Mimic HCM => resolve/revert after tx
* LVH
* LA + LV dilation
* Hypercontractility: incr SF% + velocity of circumferential fiber shortening