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InFocus

Canine hypothyroidism

David Grant continues his series looking at dermatological conditions.

MOST AUTHORITIES STATE
THAT HYPOTHYROIDISM
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
diagnostic test
(Miller, Campbell,
Griffin, 2013).

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

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

Clinical signs are divided into cutaneous
and non-cutaneous. These are variable
and present in a lesser or greater degree
in individual cases.

(A) Cutaneous

  • 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
    following clipping.
  • Thickened skin – cool to touch.
  • Hyperpigmentation and comedone
    formation.
  • Chronic seborrhoea sicca or oleosa.
  • Ceruminous otitis externa.
  • Recurrent pyoderma.
  • Non-pruritic unless secondary
    pyoderma or Malassezia.

(B) Non-cutaneous

  • Lethargy and poor exercise tolerance.
  • Weight gain without increased
    appetite.
  • Thermophilia.
  • Bradycardia.
  • CNS involvement – head tilt, cranial nerve dysfunction, diffuse peripheral neuropathy, hypermetria.
  • Prolonged anoestrus, infertility.
  • Corneal lipidosis.

Differential diagnosis

(Hnilica, 2011)

  • Other causes
    of endocrine
    alopecia.
  • Other causes
    of recurrent
    superficial
    pyoderma.
  • Malassezia dermatitis.
  • Demodicosis.

Diagnosis

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.

Haematology

  • 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.

Biochemistry

  • 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
    phosphatase.

Histopathology

Treatment

  • 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.

Thyroid testing

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
    (euthyroid sick
    syndrome).
  • Various drugs
    may also decrease
    the level of TT4.
    Glucocorticoids,
    phenobarbitone,
    phenytoin, diazepam,
    phenylbutazone,
    potentiated
    sulphonamides,
    mitotane, furosemide,
    oestrogens and
    androgens have been
    incriminated among
    others.

Free T4 (FT4)

  • FT4 measured by
    equilibrium dialysis
    is thought to be
    more specific for
    the diagnosis of
    hypothyroidism by
    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.

Thyroid stimulating
hormone (TSH)

  • In primary hypothyroidism TSH
    levels should be expected to rise to
    stimulate the production of thyroid
    hormone.
  • 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
results

(from Miller, Campbell and Grif n,
2013)

  • Consider the diagnosis of hypothyroidism to be possible if the
    TT4 or FT4 is in the low or low normal
    range.
  • 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,
    thyroid supplementation
    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
    be considered.
  • 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.
    Dermatological signs
    will need several months
    to respond.

Treatment

(From Hnilica, 2012)

  • Treatment of
    secondary problems
    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
    reference range.
  • 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.

References and
suggested reading

  • Hnilica, K. A. In: Small Animal
    Dermatology: A Color Atlas and
    Therapeutic Guide
    ; pp287-291.
    Elsevier, 2011.
  • Miller, W. H., Grif n, C.
    E. and Campbell, K. L. In:
    Muller and Kirk’s Small Animal
    Dermatology
    ; pp502-512.
    Elsevier, 2013.
  • Paterson, S. Manual of Skin
    Diseases in the Dog and Cat
    , 2nd
    edition; pp136-140. Blackwell
    Publishing, 2008.

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