Point-Of-Care in-house thyroid testing - IDEXX Laboratories

clinical applications
POINT-OF-CARE IN-HOUSE THYROID TESTING
COMPARISON OF LABORATORY
AND IN-HOUSE TESTS
THE OFFICIAL PUBLICATION OF THE NORTH AMERICAN VETERINARY CONFERENCE
SUPPLEMENT TO NAVC CLINICIAN’S BRIEF ® JANUARY 2007
WWW.CLINICIANSBRIEF.COM
POINT-OF-CARE IN-HOUSE T 4 TEST

POINT-OF-CARE IN-HOUSE T 4 TEST
2
clinical applications
POINT-OF-CARE IN-HOUSE THYROID TESTING
COMPARISON OF LABORATORY
AND IN-HOUSE TESTS
This study
demonstrates a
good correlation
between 4
different methods
of thyroxine
(T4 ) testing.
Thyroid disease is relatively common in dogs and cats;
however, dogs are most commonly hypothyroid while cats
are most commonly hyperthyroid. The measurement of total
thyroxine (T 4 ) is the mainstay for diagnosing thyroid disease in both
species and for evaluating therapeutic success. Dogs suspected of
being hypothyroid (borderline or low concentrations of T 4 ) usually
also have confirmation tests to determine free T 4 and/or thyroid
stimulating hormone (TSH or thyrotropin) concentrations. In cats
suspected of being hyperthyroid (increased concentration of T 4 ), the total T 4 is usually sufficient
to diagnose the disease, while levels in the mid to upper portion of the reference range are
considered suspicious and warrant further testing (such as measurement of free T 4 ).
Historically, radioimmunoassay (RIA) in the laboratory has been considered the gold standard
of T 4 testing, but new methods and technology advancements have made other affordable,
convenient, and time-sensitive methods available. Most RIA procedures have been designed for
people but have been validated for use in domestic animals, and a total T 4 RIA test kit is
marketed specifically for dogs. ELISA-based and chemiluminescent systems processed in
commercial laboratories
KEY POINTS – Feline & Canine Thyroid Disease
FELINE HYPERTHYROIDISM
Most often caused by
hyperfunctioning thyroid
gland nodules (secretion of T 4
and T 3 not controlled by TSH)
Common clinical signs
■ Weight loss
■ Polyphagia
■ Vomiting
■ Tachypnea
■ Hyperactivity
■ Dyspnea
Differential Diagnosis
■ Chronic kidney disease
■ Chronic liver disease
■ Neoplasia
Diagnosis
■ Total T 4 concentration
above reference range
■ High normal total T 4
coupled with elevated
free T 4
CANINE HYPOTHYROIDISM
Most common in middle-aged
dogs; acquired primary disease
(lymphocytic thyroiditis or
idiopathic thyroid atrophy)
Common clinical signs
■ Lethargy
■ Inactivity
■ Mental dullness
■ Weight gain
■ Hair loss or excessive shedding
■ Recurrent infections
Differential Diagnosis
■ Exclude other endocrine
diseases such as
hyperadrenocorticism
Diagnosis
■ Total T 4 below reference range
screening test
■ Decreased free T 4
■ Increased cTSH levels may
be seen
also have been validated for
total T 4 concentration
testing in dogs and cats. A
point-of-care in-house
ELISA system (IDEXX
SNAP ® T 4 Test) for use by
veterinarians in a practice
setting is also commercially
available. The study
presented on the opposite
page compared results
from these 4 methods of
analysis and demonstrated
a good correlation among
them. The data give the
practitioner a good basis
for selecting among
currently available T 4 testing
methods, providing a
greater range of solutions
than has been available in
the past.
c l i n i c i a n ’ s b r i e f ®

STUDY – Comparing various methods of T 4 testing 1
■ Serum samples obtained from 98 dogs and 100 cats
■ Samples were submitted to diagnostic lab to test the following:
Dogs
Screening for hypothyroidism 41
Assessing adequacy of treatment 39
Reason for request unknown 18
Cats
Screening for hyperthyroidism 64
Assess treatment with methimazole 17
Reason for request unknown 19
■ 12 pairs (24 animals) for comparison of the 4 assay methods for 2 species
■ Precision estimates were determined for each assay using serially diluted dog and cat serum
■ Samples submitted to a veterinary university endocrinology lab
■ T 4 concentration determined by RIA T 4 assay within 1 of 4 ranges
0.5 to 1.5 µg/dl 3.1 to 4.9 µg/dl
1.6 to 3.0 µg/dl 5.0 to 7.0 µg/dl
T 4 Assay Methods* †
Method Test Performed by Product maker
A Canine total T4 coated-tube RIA
Diagnostic laboratory Coat-A-Count Canine Total T4 Diagnostic Products Corp
Los Angeles, CA
B In-house enzyme- In-house (simulated)* SNAP ® linked immunosorbent
T4 IDEXX Laboratories
assay (ELISA) kit Westbrook, ME
C** RIA total T4 kit Mailed to company Coat-A-Count Total T4 Diagnostic Products Corp
Los Angeles, CA
D** CEIA kit Mailed to company Immulite Total T4 Diagnostic Products Corp
Los Angeles, CA
* Performed by a veterinary student.
† Sensitivity was 0.15, 0.5, 0.25, and 0.4 µg/dl for assays A, B, C, and D respectively.
** Marketed for human, validated for use in dogs and cats.
RESULTS
■ Statistical comparisons of T 4 test method results revealed, in general, good correlation between T 4
concentrations.
■ Agreement was generally excellent for T 4 concentrations in low to middle portion of clinically normal
ranges.
■ Divergent results were seen for all methods at higher T 4 concentrations, but it is possible that factors
such as lipemia, hemolysis, or method of serum collection contributed to these differences.
■ In general, the T 4 concentrations obtained by assay method A were lower than those of the other
methods. These differences were more profound at higher concentrations, and in cats.
Discussion
For hypothyroidism in dogs, most veterinarians use total T 4 concentrations as screening tests. Because
factors other than hypothyroidism can cause low total T 4 concentrations, confirmation tests are needed
to make a diagnosis. This study showed that a point-of-care system (method B) provided reliable and
consistent results for screening and for assessing adequacy of T 4 replacement treatment.
For hyperthyroidism in cats, there was good agreement between methods for total T 4 concentrations
above the reference range. Because some cats with hyperthyroidism have total T 4 concentrations
within reference ranges, other testing may be indicated when thyroid disease is suspected but total T 4
is within the reference range.
1. Measurement of total thyroxine concentration in serum from dogs and cats by use of various
methods. Kemppainen RJ, Birchfield JR. Am J Vet Res 67:259-265, 2006.
c l i n i c a l a p p l i c a t i o n s
3
POINT-OF-CARE IN-HOUSE T 4 TEST

4
Commentary
Canine hypothyroidism and feline hyperthyroidism are two of the most common endocrine disorders
seen in small animal practice. Because thyroid disorders typically respond well to therapy
and have a good prognosis, it is important for the practitioner to be aware of both common and
uncommon clinical signs and to routinely screen patients with suspected or potential disease.
Serum total thyroxine (T 4 ) concentrations commonly used for screening and therapeutic monitoring
are routinely offered by diagnostic laboratories using a radioimmunoassay or chemiluminescence test. The
availability of an in-house T 4 testing system can offer a convenient, cost-effective alternative. This study showed generally
good correlation between the various methodologies and it appears that any of them, including the IDEXX
SNAP ® T 4 in-house test, is appropriate, dependable, and indicated. It must be remembered that with any test, false
negative and false positive results can occur for any number of reasons, and that results must be interpreted in
light of the patient’s history, clinical signs, and other laboratory findings. — Peter P. Kintzer, DVM, DACVIM, Staff
Internist, Boston Road Animal Hospital, Springfield, Massachusetts
SUGGESTED READING
Canine hypothyroidism. Dixon RM. BSAVA
Manual of Canine and Feline Endocrinology,
3rd ed. Mooney CT, Peterson ME. Gloucester,
UK: BSAVA Publications, 2004, pp 76-94.
Feline hyperthyroidism. Mooney CT, Peterson
ME. BSAVA Manual of Canine and Feline
Endocrinology, 3rd ed. Mooney CT, Peterson
ME. Gloucester, UK: BSAVA Publications, 2004,
pp 95-111.
Hypothyroidism. Feldman EC, Nelson RW.
Canine and Feline Endocrinology and
Reproduction (3rd ed). Feldman EC, Nelson
RW. Philadelphia: WB Saunders Co, 2004, pp
86-151.
Measurement of serum concentrations of free
thyroxine, total thyroxine, and total
triiodothyronine in cats with hyperthyroidism
and cats with nonthyroidal disease. Peterson
ME, Melian C, Nichols R. JAVMA 218:529-536,
2001.
© 2007 Educational Concepts
IDEXX Product Profile
IDEXX SNAP ® Reader
■ Only in-house instrument that tests T 4, cortisol, &
bile acids
■ Easy to use
■ Attaches to top of IDEXX VetTest ® Chemistry
Analyzer, so no additional bench space needed
■ Cost effective
IDEXX SNAP ® T 4 test
■ Diagnoses thyroid
disease
■ Assesses geriatric
preanesthetic cats
■ Provides
comprehensive, onevisit
screening for cats, dogs, and horses
■ Monitors response to treatment for immediate
dosage adjustment
This research summary is sponsored by an educational grant from IDEXX Corporation.
09-65881-00
c l i n i c a l a p p l i c a t i o n s