Hypothyroidism

Hypothyroidism is a deficiency of thyroid hormones in the blood.

Primary (95%) - Destruction (immune-mediated or idiopathic atrophy) of the thyroid gland that leads to decreased secretion of thyroid hormones T3 (tri-iodothyronine) and especially T4 (thyroxine).

Secondary (5%) - Destruction of the pituitary gland, leading to decreased TSH secretion and the subsequent decrease in thyroid activity and secretion of thyroid hormones.

Iatrogenic - after hyperthyroidism treatment (most common in cats).

Symptoms

Consequence of decreased cellular metabolism. Vary, according to age

Interpretation of laboratory tests

General Tests

• Hemogram
  Moderate, non-regenerative, usually normocytic and normochromic anaemia.
• Blood Biochemistry
  A) Cholesterol reference values: Dogs: 106 - 367 mg/dL Cats: 58 - 232 mg/dL
  Vey elevated in dogs and cats.
  B) Triglyceride reference values: Dogs: 50 - 100 mg/dL Cats: 20 - 80 mg/dL
  Elevated in dogs (lipaemia).

Specific tests

T3 tests:
Not recommended since euthyroid and hypothyroid values overlap. Results vary greatly even in non-thyroid pathologies.

Total T4:
Recommended as the first-choice test, although a single test is not diagnostic. If the results are normal, it is advisable to reconsider the diagnosis. If the results are low or extremely high, it is advisable to supplement the findings with other specific tests. Determination of free T4 is somewhat more precise than that of total T4.

Interpretation:
0 - 1.17 μg/dL - Hypothyroidism, concomitant disease, or drug interference. 1.17 - 1.95 μg/dL - Inconclusive 1.95 - 6.24 μg/dL - Euthyroidism > 6.24 μg/dL - Overdose of thyroxine or immune-mediated hypothyroidism.

Free T4:

1. Reference values: Dogs: 0.6 - 2.6 mg/dL Cats: 0.6 - 2.6 ng/dL. Below-normal values in Free T4 are somewhat more indicative of hypothyroidism than similar results in Total T4. This is because Free T4 does not bind to proteins and is therefore less influenced by other pathologies.

TSH levels:
TSH secretion is stimulated in 65% - 80% of hypothyroid dogs due to decreased levels of thyroid hormone in the blood.
TSH levels are also elevated in 25% of healthy animals.
This test can be performed to supplement the findings of the T4 levels test.

Interpretation:
0 - 0.41 ng/ml - Healthy, euthyroid sick syndrome., and occasionally hypothyroid animal.
0.41 - 0.6 ng/ml - Inconclusive
> 0.6 ng/ml - Hypothyroid and occasionally, euthyroid.

TSH stimulation test (bovine):

Assesses the ability of the thyroid gland to respond to stimulus and secrete hormones. The test of choice after determining T4 levels.
Draw a blood sample.

Dog/Cat - Inject 1 IU/kg TSH iv (maximum 5 IU) Collect a second blood sample 6 hours later.

Healthy animals: Two-fold increase in T4 over baseline values and up to 2.03 - 17.2 μg/dL.
Animal with primary hypothyroidism (90%): T4 not elevated (< 2.03 μg/dL).

In cats, the increase is more moderate; some hypothyroid cats show increases while other euthyroid cats do not.

TRH stimulation test (synthetic):

Stimulates TSH secretion by the pituitary gland, which in turn increases secretion of T3 and T4 by the thyroid. This test is performed if T4 levels and the TSH stimulation test are consistent with hypothyroidism but more information is needed, or if TSH is not available for stimulation.
Draw a blood sample.

Dogs/Cats - Inject 0.1 mg/kg TRH iv. Collect a second blood sample 6 hours later.

Healthy animals: Baseline T4 is generally increased 50%.

Animals with primary or secondary hypothyroidism: T4 is not increased. T4 levels are also unchanged in 25% of healthy animals.

Antithyroid antibody test: This test can be used as a diagnostic aid.

Bibliography

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