Not so fast! Is it really hypothyroidism or something else?

Présenté par Manon Paradis, D.M.V., M. Sc. V., Dipl. ACVD

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Questions

1. You suspect a diagnosis of hypothyroidism in a dog where you have measured total thyroxine (TT4) and get a result of 10 nmol/L (reference values 15 to 45 nmol/L). What can you conclude? *

The dog is hypothyroid because the result is below normal TT4 reference values

He is not hypothyroid because the value is not low enough

He is probably euthyroid, but his TT4 is a little low as he was definitely on glucocorticoids

Nothing can be concluded

It is completely useless to measure TT4 alone because much more reliable tests are currently available

2. Sighthounds are known to have different circulating thyroid hormone concentrations. Of the following, which is the more common? *

Often underdiagnosed for hypothyroidism

Often overdiagnosed for hypothyroidism

Difficult to treat

Often show an elevated thyroid-stimulating hormone (TSH)

Always have thyroglobulin antibodies

3. Regarding canine hypothyroidism, which of the following statements is TRUE? *

Clinical signs usually appear between the age of 1 and 2 years

Most common cause is secondary hypothyroidism (pituitary origin)

In addition to skin changes, the presence of hepatomegaly and a pendulous abdomen is quite common

Thyroxine test (T4T) or free thyroxine test (FT4) combined with canine endogenous TSH is most commonly used to assess thyroid function

The treatment of choice is levothyroxine, administered for a minimum of 3 months and/or up to 1 month after resolution of clinical signs

4. Regarding canines, which of the following statements is FALSE? *

Most cases of hypothyroidism are due to lymphocytic thyroiditis

Nonregenerative anemia with a hematocrit of about 20%–25% is commonly seen during hypothyroidism

Some breeds, such as greyhounds, have lower TT4 reference values than other breeds

TT4 within reference values strongly suggests the dog is euthyroid

TT4 is often decreased to values consistent with hypothyroidism in dogs with systemic illnesses

5. Regarding canine hypothyroidism, which of the following statements is TRUE? *

Measurement of TT4 must be done using an equilibrium dialysis technique to be reliable

A screening test for congenital hypothyroidism is available through Orthopedic Foundation for Animals (OFA)

Flank alopecia and chronic or recurrent external otitis are common clinical signs

Endogenous serum thyrotropin, (cTSH ) concentration within reference limits does not rule out hypothyroidism

Most hypothyroid dogs will require thyroid hormone supplementation every 12 hours for life

6. Regarding canine hypothyroidism, which of the following statements is FALSE? *

Glucocorticoids, phenobarbital and sulfonamides are known to affect thyroid function test results

Pomeranians and miniature poodles are at high risk of developing congenital hypothyroidism

Most hypothyroid dogs are well controlled with q24h levothyroxine administration

When measuring TT4 for treatment monitoring, always verify if and when levothyroxine was administered

Avoid supplementing dogs which have nonthyroidal illness syndrome (euthyroid sick syndrome)

7. Regarding thyroid hormones, which of the following statements is FALSE? *

They have a profound effect of increasing the body’s metabolic rate

They have inotropic and chronotropic effects on the heart

They have a catabolic effect in physiological quantities

They stimulate erythropoiesis and regulate cholesterol synthesis and degradation

All circulating thyroxine (T4) and 90% of circulating triiodothyronine (T3) are produced by the thyroid glands

8. Regarding euthyroid sick syndrome in dogs, which of the following statements is FALSE? *

It is a frequent source of misdiagnosis of hypothyroidism

Causes a decrease of TT4, TT3 and an increase of rT3

Metabolic change often observed in sick animals that is believed to prevent excessive calorigenic effect of T3 in a catabolic state

Levothyroxine supplementation at half the regular dose is required for up to 3 months

Reverse T3 (RT3) is the metabolically inactive form of T3 produced in times of severe illnesses

9. Regarding congenital hypothyroidism in dogs, which of the following statements is FALSE? *

Can be primary, secondary or tertiary

A mutation in the thyroid peroxidase (TPO) gene has been identified in wire fox terriers and rat terriers

Congenital hypothyroidism with goiter in fox terriers has an X-linked dominant mode of inheritance

A genetic test is available to identify carrier dogs

Disproportional dwarfism and cretinism are common clinical signs

10. Regarding thyroglobulin, which of the following statements is FALSE? *

Large iodinated glycoprotein is synthesized and secreted by the pituitary gland

Major constituent of colloid

Serves as a synthesis and storage site for thyroid hormones and their precursors in the thyroid follicles

Each thyroglobulin molecule contains ~70 tyrosine amino acids

Major substrates that combine with iodine to form thyroid hormones