Canine hypothyroidism

David Grant continues his series looking at dermatological conditions.

01 August 2016

4 min read

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 Griffin, 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.