Hypothyroidism is one of the most commonly diagnosed hormone
disorders in the dog. Some cases are clinically very obvious while
others are more difficult to diagnose.
The causes of hypothyroidism are listed in Table 1 (pg. 4). Primary
acquired hypothyroidism is the most common type of hypothyroidism in
the dog. It is usually the result of lymphocytic (autoimmune)
thyroiditis or thyroid atrophy. Congenital hypothyroidism is rare in
occurrence. Central hypothyroidism results from damage or
dysfunction to the anterior pituitary gland. The secondary form of
hypothyroidism is rare.
Estimates of the incidence of canine hypothyroidism ranges from
1:156 to 1:500. Middle-aged dogs (4 to 10 years of age) of mid-to
large-sized breeds are at increased risk. Commonly affected breeds
include Golden Retriever, Doberman Pinscher, Irish Setter, Miniature
Schnauzer, Dachshund, Cocker Spaniel, Airedale Terrier, Great Dane
and Old English Sheepdog. German Shepherd Dogs and mixed-breeds are
at low risk.
Physiologic Effects of Thyroid
Most of the varied effects of thyroid hormones result from the
stimulation of oxygen utilization (calorigenic action) by almost all
metabolically active tissues. Tissues which do not depend on T3
for oxygen consumption include the adult brain, testes,
uterus, lymph nodes, spleen, and anterior pituitary. As a
consequence to the increased metabolic rate induced by T3
nitrogen excretion is increased, internal protein and fat stores are
broken down, and body weight is decreased. In young hypothyroid
animals, small doses of thyroid hormones induce a positive nitrogen
balance because they stimulate growth, but excessive doses will
cause protein utilization.
Large doses of thyroid hormones cause excessive body heat production
and a slight rise in body temperature. This, in turn, activates
heat-dissipating mechanics. Excessive levels of thyroid hormone in
conjunction with epinephrine secretion lead to an increased cardiac
output. In addition to these effects, thyroid hormones influence
other physiologic processes as listed in Table 2 (pg. 4).
ADVERSE SYSTEMIC EFFECTS OF
Hypothyroidism is defined as a clinical
condition characterized by inadequate quantities of circulating
thyroid hormone. Table 3 lists the various causes of hypothyroidism
in the dog. With the addition of iodized salt to commercial pet
foods, hypothyroidism is usually associated with the nongoitrous
(without cystic enlargement) form.
The clinical signs of hypothyroidism that are
related to decreased metabolic rate and calorigenesis include
lethargy, cold intolerance, decreased cerebral function, and a mild
increase in body weight. Dermatologic changes are characterized by
hair loss, skin thickening, and increased pigmentation.
Cardiovascular changes associated with a
decreased cardiac output include a weak apex beat and a weak
peripheral pulse. The electrocardiogram can show low amplitude R
waves with or without a slow heart rate. Impaired peripheral
circulation is suspected when the extremities seem cool to the
Other clinical signs associated with
hypothyroidism are decreased libido, gonad underdevelopment,
anestrus, diarrhea, constipation, anemia, muscle weakness, muscle
and nerve dysfunction (including cranial nerves VII, VIII and X
dysfunction), and mammary milk production.
Myxedema is the extreme form of
hypothyroidism. The signs in the dog are characterized by severe
mental depression terminating in coma and hypothermia. Signs of
hypothyroidism are usually present, but hypoventilation,
hypotension, slow heart rate, and profound hypothermia are usually
present as well. Often acute decompensation is triggered by an
anesthetic episode. For these reasons, great care should be taken
when anesthetizing a hypothyroid dog.
The serum T4
determination is still the most commonly run test for initial
thyroid evaluation. A normal value (1.5-4.3 μ/dl, 20-55 nmol/L)
essentially rules out hypothyroidism. A very low T4
level in conjunction with appropriate clinical signs and the
absence of confounding factors is usually sufficient for making the
diagnosis of hypothyroidism.
The serum T3
determination (normal 0.7-2.3 nmol/L, 45-150 ng/dl) is
commonly run but it is not as diagnostic as T4
measurement. It is not uncommon to find normal T3
concentrations in dogs with hypothyroidism.
The thyroid stimulating hormone (TSH) response test is used to
identify hypothyroidism when the T4
results are questionable. A reduced or no response to TSH is
expected in hypothyroidism. It is important to consider that
although the test will distinguish many borderline situations, the
results are not always clear in dogs with nonthyroid illness or
those treated with certain drugs.
However, post TSH T4
levels greater than 45 nmol/L rule out hypothyroidism while
levels less than 15 nmol/L indicate a need for replacement therapy.
On the negative side, the commercial availability of TSH is
unreliable and it is relatively expensive.
The free T4
level represents the fraction of total hormone available for
target cell entry. It comprises less than 1% of total T4.
The equilibrium dialysis technique is the preferred method with
normal values ranging from 12-33 pmol/L (Nachreiner, MSU).
Many laboratories are now offering the TSH assay. It is essential
that the appropriate standards are run with this test. Normal TSH
levels range from 7-40 mU/L (Nachreiner, MSU), while the hypothyroid
dog typically has elevated serum levels.
Antithyroid globulin antibody and antithyroid hormone antibody
titers can be increased with lymphocytic or autoimmune thyroiditis.
These dogs can be either euthyroid or hypothyroid in the presence of
an elevated antibody titer. Some dogs with elevated antithyroid
hormone antibody titers can also have increased measured serum
thyroid hormone levels while still being clinically euthyroid.
Therefore, elevated antithyroid globulin and antithyroid hormone
antibodies are not reliable tests for diagnosing canine
Primary hypothyroidism is initially treated with thyroxine (levothyroxine)
at a dosage of 22 μg/kg (0.1 mg/10lb) every 12 hours. This same dose
can be decreased to once daily treatment after the first month.
Periodic blood level monitoring should be done 4-7 hours post-thyroxine
administration, and the treatment should be adjusted accordingly.
Improved mentation and activity levels should become apparent
over the first 2-7 day period. Skin and neurological improvement
should occur after 1-3 months of treatment. Reproductive
abnormalities might hopefully improve over a 3-10 month period.
Sodium liothyronine (synthetic T3
Cytomel) is not the initial thyroid hormone supplement of
choice. While liothyronine will raise the T3
level, it will also lower the T4
level through negative feedback inhibition. Synthetic T3
therapy is indicated when levothyroxine treatment fails to
achieve a desired clinical response in a confirmed hypothyroid dog.
This can arise if there is impaired thyroxine absorption from the
The prognosis is generally good for primary hypothyroidism so
long as there are no complications associated with the
hyperlipidemia (coronary artery disease, acute pancreatitis) or the
|Table 1. Causes of Hypothyroidism in the Dog
I. Primary hypothyroidism
acquired – atrophy of unknown etiolog
autoimmune – lymphocytic thyroiditis
neoplastic – bilateral thyroid carcinoma
II. Pituitary hypothyroidism (Secondary
pituitary neoplasms, e.g. chromophobe adenoma
defective TSH molecule
III. Tertiary hypothyroidism
congenital hypothalamic malformation
acquired destruction of the hypothalamus
IV. Hypothyroidism associated
ingestin of various goitrogens, i.e.,
Table 2. Physiologic Effects of Thyroid Hormones
Effects of Excessive Levels of Thyroid Hormones
Effects of Inadequate Levels of Thyroid Hormones
rapid mentation, irritability, and restlessness
increased CNS sensitivity to circulating catecholamines
shortened stretch reflex time
increased rate of carbohydrate absorption from the intestinal
increased lipid metabolism
increased requirements of water soluble vitamins and fat
anemia resulting from decreased erythropoiesis
increased stretch reflex time
depressed cholesterol and lipid metabolism
impaired growth and skeletal maturation in young animals