MOST AUTHORITIES STATE
IS THE MOST COMMON
ENDOCRINE DISEASE IN
DOGS. It has been suggested, however,
that the disease may be over-diagnosed
and this may be due to the absence
of a single reliable
It occurs in
many breeds of
middle-aged to older dogs, although
young adult large and giant-breed dogs
are occasionally affected (Hnilica, 2011).
Causes are divided into primary,
secondary and tertiary. Of these
primary causes are the most important,
and 90% are the result of lymphocytic
thyroiditis (Paterson, 2008). The
remainder will be the result of
neoplastic destruction or possibly the
end result of lymphocytic thyroiditis.
Secondary hypothyroidism due to
a deficiency of thyroid-stimulating
hormone (TSH) and tertiary
hypothyroidism due to a deficiency of
thyrotropin-releasing hormone (TRH)
are both very rare.
Clinical signs are divided into cutaneous
and non-cutaneous. These are variable
and present in a lesser or greater degree
in individual cases.
- Bilaterally symmetrical alopecia,
particularly of the thorax, less
commonly in facial areas (Figures 1, 2
and 3). Thinning of the hair on the tail
(“rat” tail) is also possible.
- Dull brittle hair that fails to grow
- Thickened skin – cool to touch.
- Hyperpigmentation and comedone
- Chronic seborrhoea sicca or oleosa.
- Ceruminous otitis externa.
- Recurrent pyoderma.
- Non-pruritic unless secondary
pyoderma or Malassezia.
- Lethargy and poor exercise tolerance.
- Weight gain without increased
- CNS involvement – head tilt, cranial nerve dysfunction, diffuse peripheral neuropathy, hypermetria.
- Prolonged anoestrus, infertility.
- Corneal lipidosis.
- Other causes
- Other causes
- Malassezia dermatitis.
There is no one reliable test for
hypothyroidism. Clinicians must rely
on their clinical suspicion based on
the history, physical examination and
rule-out of differentials. There are a
range of investigations that support
an evaluation of the diagnosis.
These include haematology, serum
biochemistry, histopathology and
thyroid function tests.
- The most consistent finding is a
normocytic, non-regenerative anaemia,
but this is only seen in approximately
30% of the cases.
- Macrocytic and microcytic –
hypochromic anaemia may also occur.
- In between 50 and 75% of
hypothyroid dogs the serum cholesterol
and triglyceride levels are increased.
- There may be mild elevations in
serum creatine kinase and alkaline
- Histopathological examination will
demonstrate non-specific endocrine
changes and is non-diagnostic.
The presence of increased dermal
mucin, however, is suggestive of
hypothyroidism, although it is a normal finding in breeds such as the Sharpei.
Total T4 (TT4)
- A useful screening test by RIA or
ELISA for hypothyroidism. A TT4 in
or above the reference range makes the
diagnosis of hypothyroidism unlikely.
- A low TT4 is non-specific,
however, as some diseases such as
hyperadrenocorticism, liver disease,
diabetes mellitus and renal disease will decrease TT4
- Various drugs
may also decrease
the level of TT4.
androgens have been
Free T4 (FT4)
- FT4 measured by
is thought to be
more specific for
the diagnosis of
some authorities, but other studies have
shown that moderate to severe illnesses,
and some drugs, will decrease the levels below the reference range and therefore
suggesting no distinct advantage.
- In primary hypothyroidism TSH
levels should be expected to rise to
stimulate the production of thyroid
- A combination of low TT4/FT4 and
elevated TSH are highly suggestive of
hypothyroidism, but false positive and
negative results can still be possible.
- The TSH response test is potentially
very accurate. TSH is difficult to source,
however, adds considerable expense and is therefore rarely used.
A practical approach to
interpretation of thyroid
(from Miller, Campbell and Grif n,
- Consider the diagnosis of hypothyroidism to be possible if the
TT4 or FT4 is in the low or low normal
- If the TSH is elevated this adds
support to the diagnosis.
- Based on the above
the clinician has to decide
whether the clinical signs
are sufficiently compatible
to make the diagnosis.
- If the TT4/FT4 are in
the low normal range but
the TSH is not elevated,
could be administered as
a clinical trial, or the dog
could be retested at a later
time to see if values remain in the low-normal
range. At that time a
clinical trial could then
- If improvement is
to be seen during a
clinical trial, it should
occur within eight to
12 weeks. Some clinical
signs such as lethargy
and poor exercise
tolerance are often
the first to improve
during the first few
weeks of treatment.
will need several months
(From Hnilica, 2012)
- Treatment of
such as superficial
pyoderma or Malassezia dermatitis is required with appropriate topical and systemic therapy.
- Levothyroxine (0.02mg/kg orally every 12 hours) should be administered
until signs resolve. Absorption is
improved on an empty stomach.
- Ideally thereafter (to improve
compliance) the drug can be given every
24 hours, although some dogs can only
be maintained on twice-daily doses.
- Dogs with concurrent cardiac
disease, particularly cardiomyopathy,
should be started on a lower dose gradually increasing. A starting dose of 0.0005mg/kg every 12 hours is administered, increasing by 0.05mg/kg every two weeks until 0.02mg/kg is reached.
- Therapeutic monitoring can be undertaken two to four months after
initiating treatment. Serum TT4 should
be in the high normal or above range,
and TSH should be low or in the
- Signs of thyrotoxicosis (panting,
anxiety, polydipsia and polyuria) are
rare at the doses stated. If they occur,
medication should be temporarily
stopped until the signs disappear and
then re-introduced at a lower level.
- Treatment is life-long.
- Hnilica, K. A. In: Small Animal
Dermatology: A Color Atlas and
Therapeutic Guide; pp287-291.
- Miller, W. H., Grif n, C.
E. and Campbell, K. L. In:
Muller and Kirk’s Small Animal
- Paterson, S. Manual of Skin
Diseases in the Dog and Cat, 2nd
edition; pp136-140. Blackwell