Indications: For use in dogs for
correction of conditions associated with low circulating thyroid hormone
Low serum circulating L-thyroxine (T-4) concentrations,
coupled with clinical signs, are suggestive of hypothyroidism. Based on
studies conducted by Vet-A-Mix and in cooperation with two commercial
laboratories, the following parameters for T-4 concentrations in canine
serum have been established:
Normal (euthyroid) - 18 to 32 ng/mL (1.8 to 3.2 µg/dL)
Possible hypothyroid - 10 to 18 ng/mL (1.0 to 1.8 µg/dL)
Hypothyroid - less than 10 ng/mL (< 1.0 µg/dL) The
animal should be showing some clinical signs associated with
A resting serum T-4 concentration of 18 ng/mL or above
signifies that hypothyroidism is unlikely in dogs. Normally, the greater
the T-4 concentration exceeds this value, the less likely that a dog is
hypothyroid. A dog with a T-4 value below 18 ng/mL that is exhibiting
signs of hypothyroidism should be considered for levothyroxine
T-4 measurements should be made at 30 day intervals to
establish the proper maintenance dose during a therapeutic trial with
Thyro-Tabs Caplet. A critical assessment of improvement in or resolution
of clinical signs should be made after 12 weeks of levothyroxine sodium
therapy. Further confirmation of the diagnosis could include withdrawal
of the levothyroxine sodium therapy. A recurrence of clinical signs
following cessation of therapy further supports the diagnosis. Correct
diagnosis of hypothyroidism is important, since such a diagnosis
normally commits an animal to life-long supplemental L-thyroxine
replacement therapy. The principal objective of levothyroxine sodium
administration is to achieve and maintain normal metabolism in the
patient by providing an exogenous supply of synthetic L-thyroxine in
amounts sufficient to maintain levels of the hormone within the animal's
normal physiologic range. Animal adaptation may necessitate regular
monitoring of serum T-4 concentrations during the first several months
of treatment to establish proper maintenance doses.
The TSH Response Test may be used to provide a
definitive diagnosis in dogs with borderline resting serum T-4 values.
The TSH dose, post-dose sampling times, and interpretation of pre- and
post-TSH injection responses depend somewhat on the reference laboratory
Mode of actions: Levothyroxine
sodium provided by Thyro-Tabs Caplet cannot be distinguished from L-thyroxine
endogenously secreted by the thyroid gland. L-thyroxine is a naturally
circulating thyroid hormone released by the thyroid gland. The primary
regulator of thyroid function is thyroid stimulating hormone (TSH) which
is synthesized and secreted by the pars distalis of the adenohypophysis
(anterior pituitary). The mediator from the hypothalamus which exerts a
continuous influence over the pituitary release of TSH is thyrotropin-releasing
hormone (TRH). Thyroid hormones influence virtually every body organ,
either by their effect on growth and development or by the hormones'
Occurrence of canine hypothyroidism:
Hypothyroidism usually occurs in middle-aged and older dogs although the
condition will sometimes be seen in younger dogs of the larger breeds.
Neutered animals of either sex are also frequently affected, regardless
of age. The condition is usually primary, failure of the thyroid gland,
because of lymphocytic thyroiditis or other loss of follicular
epithelium and resulting atrophy of the gland. Secondary hypothyroidism
is relatively rare and usually due to a destructive pituitary tumor.
Clinical signs of canine hypothyroidism:
Not all dogs with hypothyroidism will have classical clinical signs and
laboratory findings. The following list of clinical signs and laboratory
findings may vary in dogs with hypothyroidism depending upon the degree
and length of time of the thyroid dysfunction:
Nerve and muscle function: lethargy, lack of endurance,
increased sleeping, reduced alertness and interest, impaired cerebral
function and dulled mental attitude, hypotonus, stiff and slow
movements, dragging of forelimbs, head tilt, disturbed balance.
Metabolism: decreased oxygen consumption and lower
metabolic rate, sensitivity and intolerance to cold, low body
temperature, cool skin, preference for warmth, increased body weight,
constipation, poor exercise tolerance, slow heart rate, weak pulse, weak
apex heart beat, and low voltage on ECG.
Reproduction: reproductive failure, abortion,
stillbirth, live birth of weak young, delayed puberty, reduced libido,
impaired spermatogenesis, irregular estrus and anestrus, galactorrhea.
Skin and hair: myxedema of face, blepharoptosis, atrophy
of epidermis, thickening of dermis, surface and follicular
hyperkeratosis, hyperpigmentation, coarse and sparse coat, dry, dull and
brittle hair, slow regrowth and retarded turnover of hair, bilateral
Laboratory findings: low serum T-4 concentrations,
hypercholesterolemia, hypertriglyceridemia, elevated serum creatine
kinase, anemia (normochromic, normocytic).
Dosages: The initial recommended
daily dose is 0.1 to 0.2 mg/10 pounds (4.5 kg) body weight in single or
divided doses. Dosage is then adjusted by monitoring the T-4 blood
levels of the dog every four weeks until an adequate maintenance dose is
established. The usual daily maintenance dose is 0.1 mg/10 lb (4.5 kg).
A maximum of 0.8 to 1.0 mg total daily dose will be sufficient in most
dogs over 80 pounds in body weight.
Administration: Thyro-Tabs Caplet
may be administered orally or placed in the food.
Adverse Reactions: There are no
specific adverse reactions associated with levothyroxine sodium
administration at the recommended dosages. Overdosage will result in the
signs of thyrotoxicosis listed above under cautions.
Caution: Federal law restricts this
drug to use by or on the order of a licensed veterinarian.
sodium therapy is contraindicated in thyrotoxicosis, acute myocardial
infarction, and uncorrected adrenal insufficiency. Other conditions in
which the use of L-thyroxine replacement therapy may be contraindicated
or should be instituted with caution include primary hypertension,
euthyroidism, and pregnancy.
Warnings: The administration of
levothyroxine sodium to dogs to be used for breeding purposes or in
pregnant bitches has not been evaluated. There is evidence to suggest
that administration to pregnant bitches may in some instances affect the
normal development of the thyroid gland in the unborn pups.
Precautions: The clinical effects of
levothyroxine sodium therapy are slow in being manifested. Overdosage of
any thyroid drug may produce the signs and symptoms of thyrotoxicosis
including, but not limited to: polydipsia, polyuria, polyphagia, reduced
heat tolerance and hyperactivity or personality change. Thyro-Tabs
Caplet 0.1 mg and 0.7 mg caplets contain FD&C Yellow #5 (tartrazine)
which has been associated with allergic-type reactions (including
bronchial asthma) in susceptible humans. It is unknown whether such a
reaction could also occur in other animals.
How Supplied: Thyro-Tabs (levothyroxine
sodium tablets, USP) is available as scored, color-coded caplets in 9
concentrations: 0.1 mg -yellow; 0.2 mg - pink; 0.3 mg green; 0.4 mg -
maroon; 0.5 mg - white; 0.6 mg - purple; 0.7 mg - orange; 0.8 mg - blue;
1.0 mg - tan; in 28-day strip packs and bottles of 120 and 1000.
Storage: Store at controlled room
temperature: 15°-30°C (59°-86°F) and protect from light.