Canine Hypothyroidism

Sushant Sadotra

Introduction:

Hypothyroidism is a prevalent thyroid disorder that is caused due to the deficiency of thyroid hormone. In this condition, there is an irregular short production and improper secretion of thyroid hormones into the blood from the thyroid gland. This leads to a slow metabolic rate and loss of proper body functions. Regarding pet animals, hypothyroidism is mostly occurring among dogs and rarely in cats and other pet animals.

Hypothyroidism Etiology in canines:

Imbalance at any level of the hypothalamic-pituitary-thyroid axis can cause hypothyroidism in animals, especially dogs. In Adult dogs, the onset of primary hypothyroidism can cause either because of lymphocytic thyroiditis or idiopathic atrophy of the thyroid gland. The gradual destruction of follicles and secondary fibrosis on the gland because of diffuse infiltration by lymphocytes, plasma cells, and macrophages into the thyroid gland. This condition is known as lymphocytic thyroiditis. Loss of thyroid parenchyma leads to the idiopathic atrophy of the thyroid gland, in which fatty tissue will replace the lost thyroid parenchyma. The condition can occur due to autoimmune thyroiditis. Secondary hypothyroidism can be caused due to the damage of pituitary thyrotrophs by tumor growth, leading to the deficiency of one or more pituitary hormones. Hypothyroidism in dogs can occur due to other rare causes such as congenital hypothyroidism or neoplastic destruction of thyroid tissue. Congenital hypothyroidism can be primary or secondary. Thyroid dysgenesis and dyshormonogenesis can cause congenital primary hypothyroidism in dogs. Congenital secondary hypothyroidism can show clinical signs caused by the deficiency of growth hormones such as dwarfism, lethargy, gait abnormalities, or pituitary dwarfism.

Diagnosis of hypothyroidism in canines:

A variety of nonthyroidal factors and other conditions can mimic thyroid disorder and mislead the correct diagnosis of canine hypothyroidism. The severity and chronicity of the clinical findings associated with hypothyroidism and other clinicopathologic abnormalities of the hypothyroid state can be a basis for the choice of a proper diagnostic test. Tests that could confirm the diagnosis of hypothyroidism in canines are mentioned below:

1. Total T4: The initial screening test for hypothyroidism can determine Total T4 concentration. A dog with a T4 concentration lower than the reference range limits may be anticipated to have a hypothyroid issue. However, it can also indicate that the dog can have a nonthyroidal illness such as sick euthyroid syndrome. Therefore, the total T4 test alone cannot be a proper diagnosis.
2. Free T4: In serum, unbound T4 is supposed to be biologically active. Therefore, it is a very used test to differentiate between hypothyroid and euthyroid dogs. Also, a free T4 assay can give a better diagnosis with high sensitivity and specificity. Most commercial Free T4 tests use the method of single-stage solid phase (analogue) assays. However, an equilibrium dialysis step can improve the accuracy.
3. Free T3: Among T4 and T3, T3 is the most potent hormone in animals. Therefore, measuring Free T3 is also a sensitive diagnostic step. However, during the onset of the primary hypothyroidism, serum T3 determination is weak. Also, it is found that in hypothyroidism dogs, serum T3 concentrations could be low, normal, or high. If the serum concentration of T3 is high, it is to be checked whether or not anti-T3 antibodies are producing false results in the T3 radioimmunoassay.
4. Serum TSH concertation: In primary hypothyroidism, high serum TSH concentrations are expected that further may lead to low serum concentrations of T4 and free T4. A species-specific TSH assay can be used to check the high level of serum TSH. However, a false-negative result showing normal TSH concentrations may indicate a condition of primary hypothyroidism; a false-positive result showing high serum TSH concentrations may indicate a nonthyroidal illness in a euthyroid dog. A normal serum TSH concentration can indicate secondary hypothyroidism in a few cases. Therefore, serum TSH concentration results should always be coupled with other tests for definite confirmations.
5. TSH stimulation test: In this particular test, bovine TSH is introduced exogenously into the dog’s body, and the thyroid gland’s response is evaluated. Firstly, a basal T4 is measured. Then bovine TSH is administrated at a dosage of 0.1 U/kg. After 6 hours, T4 levels are calculated for the second time to check the response of the thyroid gland. Result interpretations could be a no response for hypothyroidism, a normal and blunted response for the sick euthyroid syndrome. Although the TSH stimulation test is one of the accurate tests to check thyroid function, it is expensive and less available.
6. Imaging: Thyroid Ultrasonography can detect the decreased echogenicity followed by decreased thyroid volume. The procedure can take the best imaging of the thyroid gland by technetium 99m (99mTc). This diagnostic tool can differentiate between hypothyroidism and euthyroid sickness.
7. Therapeutic trial: In this approach, a thyroxine supplementation is given to a dog at a particular dosage. If the response is positive, the supplementation is stopped to check for the return of clinical signs related to hypothyroidism. This can confirm that the dog has thyroid-responsive diseases rather than other nonthyroidal issues. However, before starting a therapeutic trial, every attempt should eliminate nonthyroidal sickness. Also, therapeutic monitoring should be performed in case the therapy is unsuccessful.

Treatment of hypothyroidism in canines:

The typical treatment of hypothyroidism in dogs is the oral medication of levothyroxine (L-T4). L-T4 is a synthetic thyroid hormone that can restore blood thyroid hormone concentrations and reverse hypothyroidism’s effects. The replacement of natural with synthetic hormones will be used for the rest of the animal’s life. However, a great precaution is needed with the initial dose and tailoring of the drug. The dosages of L-T4 in d are 0.01–0.02 mg/lb (0.02–0.04 mg/kg). The drug is given every day once or twice without food.

Reference:

1. Hypothyroidism is the Most Common Hormone Imbalance of Dogs; Wendy Brooks, DVM, DABVP. Date Published: 02/26/2002, Date Reviewed/Revised: 08/16/2019.
2. Strey S, Mischke R, Rieder J. Hypothyreose beim Hund: eine Übersicht [Hypothyroidism in dogs: an overview]. Tierarztl Prax Ausg K Kleintiere Heimtiere. 2021 Jun; 49(3):195-205. German. doi: 10.1055/a-1367-3387. Epub 2021 Jun 22. PMID: 34157761
3. Mark E. Peterson. Hypothyroidism in Animals. Last full review/revision Jul 2019 | Content last modified Oct 2020. MSD MANUAL Veterinary Manual.