An update on feline hypothyroidism

By Marie A. Chartier, DVM, DACVIM

Hypothyroidism is an uncommon disease in the cat. The most common cause of feline hypothyroidism is the iatrogenic form. Other less common forms include naturally acquired adult-onset primary hypothyroidism and congenital primary hypothyroidism. Differentiating hypothyroid from euthyroid sick cats can be challenging and usually requires multiple diagnostics test.

Once a definitive diagnosis of hypothyroidism is established, treatment with thyroid supplementation can often lead to complete clinical improvement. Monitoring therapy through recheck thyroid hormone concentrations is recommended for long-term control of the disease.

Iatrogenic hypothyroidism

Iatrogenic hypothyroidism is the most common form of feline hypothyroidism. Iatrogenic hypothyroidism can be secondary to bilateral thyroidectomy, radioactive iodine therapy, or anti-thyroid medication. Previous studies have reported a 30% incidence of hypothyroidism after radioactive iodine (¹³¹I) treatment and a 35% incidence of hypothyroidism after anti-thyroid medication treatment. Hypothyroid cats were more likely to become azotemic, and survival times were shorter in cats that were both hypothyroid and azotemic.^1,2 Iatrogenic hypothyroidism may be more common than previously reported and ongoing studies are looking at the prevalence of overt and subclinical hypothyroidism after ¹³¹I therapy. These cases can be difﬁcult to diagnosis as many cats can take months to develop clinical signs and in numerous cases only mild to subclinical signs are present. Idiopathic hypothyroidism must be differentiated from “euthyroid sick syndrome” and from the cases with a transient decrease in total T4 that can be seen within the ﬁrat month of ¹³¹I therapy. Transient hypothyroidism usually resolves over 3-6 months as normal thyroid tissue returns to function. Clinical signs in cats with iatrogenic hypothyroidism are similar to canine hypothyroidism and can include lethargy, inappetence, obesity, and dermatologic changes. A mild progressive azotemia can be seen in hypothyroid cats due to a lower glomerular ﬁltration rate (GFR). Progressive hypercholesterolemia, a non-regenerative anemia, and an elevated creatinine kinase are not uncommon.

Adult-Onset Hypothyroidism

Naturally acquired adult-onset primary hypothyroidism is rare. This condition has been documented in a few case reports. Lymphocytic thyroiditis was diagnosed in one cat.^3,4 The thyroid gland was not identiﬁed in the other cat. The clinical signs appear to be similar to cats with iatrogenic hypothyroidism. Palpable goiter can be present. Facial myxedema has been reported in one cat.⁴ Bradycardia and mild hypothermia can also be seen.

Congenital Hypothyroidism

Congenital primary hypothyroidism in the cat is more common than adult-onset hypothyroidism. It has been reported to cause disproportionate dwarﬁsm leading to growth retardation, a large head, a short, broad neck, and short limbs. Other clinical signs can include lethargy, mental dullness, constipation, hypothermia, and bradycardia. Affected kittens are usually normal at birth, but growth retardation is noted by 6-8 months of age.

Diagnostics

Serum T4 Concentration: Serum total T4 concentration is a common screening test for thyroid disease in the cat. Most cats with hypothyroidism will have a baseline serum T4 concentration below the reference range. A serum T4 concentration at the low end of the reference range can also be seen with mild or subclinical hypothyroidism. Nonthyroidal illness and glucocorticoids can falsely lower the T4 concentration.

Serum T3 Concentration: Serum total T3 concentrations are uncommonly used to support hypothyroidism. Unlike the dog, cats with hypothyroidism tend to have concentrations that correlate with serum total T4. A normal serum T3 does not rule out hypothyroidism. Nonthyroidal illness and glucocorticoids can suppress serum T3 concentration.

Serum Free T4 Concentration: Serum FT4 is a measure of the circulating, biologically active form of thyroid hormone. A low serum FT4 supports a diagnosis of hypothyroidism, but it is possible for a cat with mild or subclinical hypothyroidism to have a result at the low end of the reference range. A small percentage of hypothyroid cats can have higher results. Although there is less inﬂuence on serum FT4 compared to serum total T4 in nonthyroidal illness, nonthyroidal illness can lower the free T4 concentration.

Serum Thyroid-Stimulating Hormone Concentration (TSH): There is not a specific assay available for measuring feline TSH concentration. The commercially available canine TSH assay has been validated for use in cats and should be utilized in cases of suspected hypothyroidism. The sensitivity of the assay alone is poor. A high TSH concentration combined with a low total T4 increases sensitivity and is highly speciﬁc for hypothyroidism. Serum thyroid panels, which include serum total T4, FT4, total T3, and TSH, can be used in combination with history and clinical signs when making a deﬁnitive diagnosis of feline hypothyroidism.

Other Diagnostics: Thyroid scintigraphy has been used to distinguish hypothyroid dogs from euthyroid sick dogs. This has not been evaluated extensively in cats. The thyroid-stimulating hormone stimulation test has been shown to be a useful diagnostic for a diagnosis of hypothyroidism but is rarely used due to the cost of the recombinant human thyrotropin needed to perform the test.

Treatment

If a low to low-normal serum total T4 is noted after treatment with an anti-thyroid medication, treatment through dose reduction should be initiated. Thyroid hormone supplementation is the treatment of choice in cats with spontaneous hypothyroidism or iatrogenic hypothyroidism secondary to ¹³¹I or surgical thyroidectomy. The biggest challenge in these cases is making a deﬁnitive diagnosis of hypothyroidism. Cats with overt hypothyroidism indicated by low serum concentrations of T4, FT4, T3 and an elevated TSH should be treated. In cats with subclinical hypothyroidism indicated by low normal serum concentrations of T4, FT4, T3 and an elevated TSH, treatment is often dependent on clinical signs and the level of azotemia. If hypothyroidism is thought to be contributing to progressive azotemia, treatment should be initiated. If no clinical signs or azotemia are present, monitoring through recheck thyroid hormone and TSH concentrations is often elected. Synthetic sodium levothyroxine (L-T4) is the supplementation of choice in dogs and cats. Tablet and liquid forms are available. The half-life of L-T4 is shorter in cats than it is in dogs. Many cats have been shown to have poor absorption of the drug. Based on these ﬁndings, giving this medication on an empty stomach and twice daily dosing are recommended. An initial dose of 75µg BID is recommended. If once daily dosing is elected, 150µg is an adequate starting dose.

Monitoring should be accomplished through recheck serum hormone and TSH concentrations 2-4 weeks after dose adjustments are made. A complete history and physical examination, including an accurate body weight, are important to have before making medication adjustments. Renal values should also be monitored for mild progressive azotemia in cats that are overtly hypothyroid. Post-treatment peak blood samples should be collected 4 hours after the morning pill. Trough levels may be elected if a once-daily dosing regimen is elected.

Prognosis

The prognosis for iatrogenic and adult-onset hypothyroidism can be good with appropriate supplementation and monitoring. If serum hormone concentrations are normal after 6-8 weeks of treatment but clinical signs are still present, further diagnostics for an underlying disease process should be considered. The prognosis for congenital hypothyroidism can be variable. Thyroid hormone supplementation often results in resolution of clinical signs, but long- term musculoskeletal and neurologic problems are common.