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Triiodothyronine (T 3 ) Troponins, Cardiac 1389

in the effusion that are greater than serum • Marked hypertriglyceridemia can predispose AUTHOR: Ruanna E. Gossett, DVM, PhD, DACVP
triglyceride levels indicate chylous effusion, to pancreatitis in dogs, so dietary intervention EDITOR: Lois Roth-Johnson, DVM, PhD, DACVP
VetBooks.ir
is warranted.
even if the effusion is not opaque white.

Triiodothyronine (T 3)

Definition Next Diagnostic Steps to Consider Specimen Collection and Handling
Active form of iodine-containing thyroid if Levels are High Serum (red top tube), 1 mL. To avoid interfer-
hormone; primary function is to increase • If hyperthyroidism suspected: clinical evalu- ence by fibrin, allow full clot formation before
rate of cell metabolism. Rarely measured as ation (p. 503) centrifugation and removal of serum. Store
diagnostic test • If autoantibodies suspected: measure T 3 refrigerated up to 7 days. Frozen samples are
autoantibody levels. stable for up to 2 months.
Synonym • Evaluate diet and supplements for iodine
3,5,3′-triiodothyronine content, raw foods. Relative Cost: $
Physiology Causes of Abnormally Low Levels Pearls
Approximately 20% of T 3 is produced in the Spontaneous or iatrogenic hypothyroidism; • T 3 is of little value in diagnosis of hypothy-
thyroid gland; the rest is from conversion non-thyroidal illness; drugs roidism due to broad overlap in T 3 serum
of T 4 to T 3 in cells. Thyroxine (T 4 ) is 99% levels between euthyroid, hypothyroid,
protein-bound in circulation. T 4 dissociates to Next Diagnostic Steps to Consider and euthyroid sick dogs (insensitive and
the more biologically active free T 3 to enter if Levels are Low nonspecific).
cells. Binding to intracellular receptor proteins Measure TSH and T 4 or free T 4 by equilibrium • Serum T 3 decreases later in the course of
causes an increased metabolic rate. dialysis; evaluate for non-thyroidal diseases and hypothyroidism than T 4 .
drug administration.
Reference Interval AUTHOR: Shelley Burton, DVM, MSc, DACVP
Dogs and cats: 0.5-1.8 ng/mL. Unit conversion: Drug Effects EDITOR: Lois Roth-Johnson, DVM, PhD, DACVP
1 ng/mL × 1.536 = 1 nmol/L (SI unit). Many drugs, including glucocorticoids and
phenobarbital, decrease T 3 concentration. Not
Causes of Abnormally High Levels as thoroughly studied as for T 4 , but consider
Hyperthyroidism; presence of autoantibodies that many drugs may have this effect.
Laboratory Tests
to T 3 ; exogenous T 3 , TSH, or TRH; rarely,
administration of iodine-containing compounds

Troponins, Cardiac

Definition because they are antigenically distinct from • Cats: 0.03-0.16 ng/mL (cTnI), 0.05-
Troponins are intracellular myofibrillar proteins skeletal muscle troponins. Cardiac troponins 0.22 ng/mL (cTnT)
expressed in cardiac and skeletal muscle. Cardiac are released into the circulation during
troponin I (cTnI) is the most diagnostically cardiac ischemia and necrosis. They increase Causes of Abnormally High Levels
relevant. in proportion to the degree of myocardial • Dogs: pericardial effusion caused by hem-
injury, regardless of cause. Troponins increase angiosarcoma or idiopathic pericarditis
Physiology in circulation within 4-6 hours after myocyte (serum levels greater with hemangiosarcoma,
• Troponins regulate the contractility of injury and may persist up to 1 or 2 weeks; though substantial overlap), arrhythmogenic
striated muscle. They mediate the interac- renal function affects clearance. They are right ventricular cardiomyopathy of boxers,
tion between the sarcomeric proteins actin sensitive and specific markers of myocardial gastric dilation/volvulus, doxorubicin car-
and myosin. The troponin complex is injury. diotoxicosis, babesiosis, acute pulmonary
composed of three proteins: troponin I is thromboembolism
the inhibitory component that prevents Reference Interval • Dogs and cats: myocardial contusion, myo-
interaction between actin and myosin until Different diagnostic instruments assess different carditis, arrhythmias, structural heart disease
intracellular calcium is bound by troponin parts of the cTnI molecule such that normal associated with congestive heart failure
C; troponin T is the tropomyosin-binding reference ranges are instrument-specific. Contact • Cats: hyperthyroidism, hypertrophic cardio-
element that binds troponin I to the actin laboratory for reference range information. myopathy
filament. Sample normal ranges:
• The cardiac isoforms of troponins I and T • Dogs: < 0.2 ng/mL (cTnI), 0.01-0.15 ng/
are specific markers for myocardial injury mL (cTnT), 0.05-0.21 ng/mL (cTnC)

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