Is Hypothyroidism Overdiagnosed in Dogs?

In the Literature

Travail V, Sanchez CF, Costo JM, et al. Assessment of the likelihood of hypothyroidism in dogs diagnosed with and treated for hypothyroidism at primary care practices: 102 cases (2016-2021). J Vet Intern Med. 2024;38(2):931-941. doi:10.1111/jvim.16993

The Research …

Canine hypothyroidism is a commonly diagnosed endocrinopathy and increasingly screened for in dogs without signs of hypothyroidism due to inclusion of total thyroxine (tT₄) measurement on complete blood work panels from reference laboratories. Increased screening increases the risk for false-positive results and incorrect diagnosis due to the 73% to 82% specificity of tT₄ measurement.¹ False-positive results may be caused by euthyroid sick syndrome from a nonthyroidal illness (NTI) that prompts blood work evaluation or administration of medications (eg, prednisone, carprofen) that can decrease tT₄ concentration.¹

In this study, records of dogs (n = 102) prescribed levothyroxine at general practice clinics underwent a blind, independent review by 3 internal medicine specialists to determine the likelihood of true hypothyroidism. All dogs had tT₄ measurement, with 89.2% of results below and 10.8% within the reference interval. In 28.4% of cases, tT₄ was measured as part of routine clinicopathologic testing and not specifically requested. An in-clinic analyzer was used to measure tT₄ in 23.5% of cases.

The specialists considered supplementation not indicated in 58.8%, 52.9%, and 45.1% of cases. Two specialists identified a higher proportion of cases in which levothyroxine was considered not indicated when tT₄ was measured with an in-clinic analyzer. All specialists identified a higher proportion of cases in which levothyroxine was considered not indicated when tT₄ was included on a blood panel but not specifically requested. The most common explanation of why levothyroxine was not indicated was lack of clinical signs and clinicopathologic findings consistent with hypothyroidism.

… The Takeaways

Key pearls to put into practice:

• Hypothyroidism is a common endocrinopathy in dogs but is likely overdiagnosed. Misdiagnosis can result in chronic, unnecessary treatment and expense for medication and repeat testing.

• tT₄ concentrations below the reference interval on routine blood work should be interpreted with caution. Clinical signs and clinicopathologic findings (eg, hypercholesterolemia) consistent with hypothyroidism should be identified prior to consideration of levothyroxine therapy. Dogs without supportive clinical findings can be retested in 3 months if indicated.

• tT₄ concentrations below the reference interval in dogs receiving medications (eg, prednisone, carprofen) that can cause low tT₄ and low free T₄ concentrations should be interpreted with caution.¹ Retesting after discontinuation of medication should be considered if possible.

• tT₄ concentrations below the reference interval in dogs with NTI should be interpreted with caution. Retesting after resolution of NTI should be considered. Testing free T₄ and thyroid-stimulating hormone concentrations can also be considered; however, free T₄ may be reduced due to NTI, and thyroid-stimulating hormone may be increased during recovery.¹

• tT₄ concentrations measured with in-clinic analyzers should be interpreted with caution, as results may not correlate well with measurements via other methodologies.²