3346. Primary Hypothyrodism in a 4-year-old female spayed dog.

Jun 19, 2019

TP 
Case study
Emaciated 4-year-old X-bred, F, Spayed vomiting.

Jun 14, 2019. Blood test
Total Cholesteroal  normal
Triglycerides normal
Glucose (Fasting)  normal 
Liver normal
Kidney  
   Urea  79.7 mmol/L   (4.2-6.3)
   Creatinine  436 umol/L  (89-177)
Haemoglobin, Red cell count, Total white cell count are normal. 


Jun 14, 2019. Serum test
     Total T4    9.19 nmol/L  (13-52).   LOW
     TSH          0.40 ng/mL    (9- 0.40)
     Free T4     4.02 pmol/L   (7-40)     LOW

1.  Possibly primary hypothyroidism but need to consider the dog's history, physical examination and other laboratory test. 
2.  Autoimmune Thyroiditis due to autoimmune disease.

Most common in dogs 4-10 years old. More than 95% of canine hypothyrodism is due to the destruction of the thyroid gland (primary hypothyroidism). The 2 most common causes of adult-onset primary hypothyroidism in dogs include lymphoycytic thyroiditis and idiopathic atrophy of the thyroid gland. Congenital primary (juvenile-onset) hypothyroidism (thyroid dysgenesis, athyreosis)  and neoplasia are rare forms of hypothyroidism. Spayed females have a higher risk compared to intact female dogs.

Usually lethargy and weight gain, heat-seeker due to frank hypthermia. Retarded regrowth of hair. Usually bilateral truncal alopecia or hair-thinning.

TREATMENT
Replacement dosages for T4 in dogs range from a total dose of 0.02 - 0.04 mg/kg, daily, given once or divided bid without food (on empty stomach).

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Hair Cycling Abnormalities

Hypothyroidism

Clinical Signs

No sex predilection, but neutered are at greater risk, and 6–10 years > risk. Onset of disease begins earlier (2–3 years old) in large/giant breeds. It is known as the 'great imitator' due to the wide variety of clinical signs that may be manifested.

Classic cutaneous clinical signs are:

1.  Bilaterally symmetric truncal alopecia (spares extremities) - hypotrichosis in frictional areas first. Large/giant breeds lose hair on lateral surface of extremities. Advanced cases loss of all but head and distal extremities.

2.  Dull, dry, brittle, easily removed hair coat - fails to regrow post clipping.

3.  Mucin accumulation. This can lead to myxedema (thick, puffy skin that's cool to touch) - most prominent on the face "tragic" expression.

4.  Hyperpigmentation

5.  Seborrhoea - Commonly seen due to the effects of thyroid hormone on serum and cutaneous fatty acid concentrations. Clinically it may present as dryness, greasiness, or seborrhoeic dermatitis.

6.  Susceptibility to skin infections.

7.  Lack of pruritus. Generally hypothyroidism is not a pruritic disease, unless complicated by secondary infections (bacteria/Malassezia).

Unusually, hypertrichosis (due to retarded turnover of hairs) may be seen, particularly in Boxers and Irish setters. The coat may become lighter in colour because the retained hairs are bleached.

Other changes that may be seen include:

1.  Ceruminous otitis or the medial pinnae and canals become dry and scaly. The changes may predispose the ears to Malassezia and/or bacterial infections.

2.  Bacterial infections

3.  Poor wound healing and easy bruising

4.  Comedone formation - usually over the ventral abdomen

Noncutaneous Cx

There is a wide range of clinical signs affecting the CNS; neuromuscular, gastrointestinal and cardiovascular systems; ocular changes; changes to CBC and reproduction (along with heat-seeking behavior); and weight gain despite unchanged dietary input.

NB hypothyroidism can occur concurrently with other endocrinopathies, be secondary to others (e.g., Cushing's), or be part of a polyglandular problem (e.g., diabetes mellitus, hypoadrenocorticism and thyroiditis occur due to autoimmune disease).

Diagnosis

Hypothyroid dogs normally present with a gamut of clinical signs. There are some important clues in identifying a suspected hypothyroid dog: middle aged, recurrent pyoderma in older animals that responds completely to antibiotics (NB some atopics will also present in a similar manner; however, it is generally in a younger dog), a history of heat seeking, lethargy, weight gain whilst on a constant level of feeding, poor libido or abnormal cycling - these all raise the index of suspicion.

Because of the variety of clinical signs and the fact no one test is definitive, a range of tests may be necessary:

 Haemogram

 Normocytic normochromic, nonregenerative anaemia is seen in 30% of cases due to folic acid, B12 metabolism deficiency.

 Microcytic hypochromic anaemia may occasionally be seen due to iron metabolism deficiency.

 Microbiological assays

 Hyper-cholesterol (50–75% of cases), CPK elevation (< 50% of cases), urine analysis - normal.

 Thyroid Testing

 Total T4 (TT4), Total T3 (TT3)
The evaluation of baseline total T3 and total T4 has been subject to controversy, as the levels of each will fluctuate during the day and are affected by numerous factors other than thyroid function alone. Added to this is the fact that the tests themselves are imprecise with varying rates of specificity and sensitivity.
A positive test in the absence of supportive clinical signs does NOT equal a diagnosis of hypothyroidism.

 Free T4 (fT4)
More consistent measure - because fT4 determines availability to the cells and TT4 may change (in response to illness, drugs, etc.) without changing fT4.

 Serum TSH Measurement
This is the most reliable method in humans. The combination of low fT4 together with elevated endogenous thyroid-stimulating hormone (TSH) seems to be the most reliable method currently available.

 Function Studies

 TSH Stimulation
Vastly superior to basal hormone determination. Bovine TSH is no longer commercially available; however, recent pilot studies have suggested that human TSH may be a viable alternative. This still requires further validation before it can be recommended.

 TRH Stimulation
This test is unreliable (Frank 1996).

 Histopathology
Generally, the changes are nonspecific, but suggestive changes include vacuolated hypertrophied arrector pili muscle, increased dermal mucin, thick dermis. The other use of the biopsy is to rule out other differentials, e.g., Demodex, infections, follicular dysplasia.