The evaluation of thyroid function is one of the most important steps in the investigation of endocrine disorders in routine veterinary practice. However, the correct interpretation of hormone tests does not depend solely on laboratory methodology; it begins even before sample collection, during the so-called preanalytical phase.
Among the factors involved in this stage, the patient’s medication history is one of the most important—and yet one of the most frequently overlooked—in clinical practice. Several commonly used medications can interfere with the function of the hypothalamic-pituitary-thyroid (HPT) axis and temporarily alter hormone levels, even in the absence of primary thyroid dysfunction.
Ignoring this variable can lead to misdiagnoses, unnecessary treatments, and reevaluations that could be avoided with a properly conducted medication history.
Why is the pre-analytical phase critical in thyroid testing?
The preanalytical phase encompasses everything that precedes the analysis itself: patient preparation, fasting, time of collection, sample storage conditions, and, especially, medication use.
In the case of thyroid function, this phase takes on even greater importance: the HHT axis is sensitive to physiological and pharmacological changes, and various medications can alter the synthesis, metabolism, binding to carrier proteins, or secretion of thyroid hormones. As a result, test results may be abnormal even in euthyroid patients.
How do medications affect the thyroid axis?
In general, medications can interfere with thyroid function through various mechanisms, including:
- Suppression of pituitary TSH secretion;
- Decreased synthesis or release of thyroid hormones;
- Displacement of the compound from its binding to carrier proteins, altering the free circulating fraction;
- Increased metabolism and hepatic clearance of thyroid hormones.
These changes are usually temporary and reversible after discontinuation of the medication; however, if they are not taken into account during interpretation, they may be mistaken for true hypothyroidism—which underscores the importance of always correlating laboratory results with the patient’s clinical and medication history.
Medications associated with interference in thyroid tests:
| Medication | Predominant effect | Mechanism involved |
| Glucocorticoids | Decrease in TT4 and TSH | Suppression of the hypothalamic-pituitary-thyroid axis |
| Potassium bromide | Decrease in TT4 | Interference with iodine uptake by the thyroid |
| Phenobarbital | Reduction in TT4 and T4L | Increased hepatic metabolism of thyroid hormones |
| Sulfonamides | Decreased TT4 and T4L levels and a possible increase in TSH | Inhibition of thyroid hormone synthesis |
| Propranolol | Reduction in T4L (peripheral conversion) | Inhibition of the conversion of T4 to T3 |
| Carprofen | Slight decrease in TT4 | Displacement of the compound from carrier proteins |
| Meloxicam | Slight decrease in TT4 | Displacement of the compound from carrier proteins |
| Ketoprofen | Slight decrease in TT4 | Displacement of the compound from carrier proteins |
The severity of these changes varies depending on the dose, duration of use, and the patient's clinical condition, and is not always accompanied by clinical signs of thyroid dysfunction.
Drug-induced interference or sick euthyroid syndrome?
In addition to the direct effects of the drugs, it is important for the veterinarian to distinguish between drug-induced interference and sick euthyroid syndrome—a condition in which systemic, non-thyroidal diseases (renal, hepatic, neoplastic, infectious, among others) also reduce total T4 concentrations as part of the body’s response to the underlying disease.
In both cases, the challenge is the same: to prevent a secondary abnormality from being misinterpreted as primary hypothyroidism. Differentiation depends on a comprehensive analysis of the clinical presentation, medication history, and comorbidities, as well as—when indicated—repeating the test after the systemic condition has resolved or the medication has been discontinued.
How can we conduct research more safely?
A few practical precautions can help reduce the impact of drug interference on the interpretation of test results:
- List all medications currently being taken, including anti-inflammatory drugs and anticonvulsants, before requesting hormone tests;
- Whenever clinically feasible, consider the most appropriate time for sample collection in relation to the start or discontinuation of the medication;
- Interpret abnormal results in light of the patient's overall clinical picture, rather than in isolation;
- Repeat the hormonal evaluation after concomitant conditions have been resolved or medication has been adjusted, when there is diagnostic uncertainty.
These simple measures, implemented as early as the preanalytical phase, significantly reduce the risk of misdiagnoses and unnecessary treatment.
The Role of Technical Support in Interpreting Test Results
Given the multitude of factors that can influence thyroid function tests, careful interpretation of the results is just as important as the analytical quality of the test itself. Hormonalle provides specialized technical support to help veterinarians correlate clinical history, medication use, and laboratory results, contributing to a more accurate diagnosis of endocrine disorders.
If you have any questions about potential drug interactions in a specific case, our technical team is available to assist with interpretation based on the patient’s clinical context.
Bibliographic References
FELDMAN, E. C.; NELSON, R. W.; REUSCH, C. E.; SCOTT-MONCRIEFF, J. C.; BEHREND, E. N. Canine and Feline Endocrinology. 5th ed. St. Louis: Elsevier, 2025.
MOONEY, C. T.; PETERSON, M. E. BSAVA Manual of Canine and Feline Endocrinology. 4th ed. Gloucester: BSAVA, 2012.
DIXON, R. M.; MOONEY, C. T. Evaluation of Serum Free Thyroxine and Thyrotropin Concentrations in the Diagnosis of Canine Hypothyroidism. Journal of Small Animal Practice, 1999.
NELSON, R. W.; COUTO, C. G. Small Animal Internal Medicine. 6th ed. St. Louis: Elsevier, 2020.
Why can medications affect the results of thyroid tests?
Several commonly used medications interfere with the hypothalamic-pituitary-thyroid axis, temporarily altering hormone levels even in the absence of actual thyroid dysfunction. This can cause a euthyroid patient (with a normal thyroid) to have abnormal test results.
Which medications most commonly interfere with thyroid tests in dogs?
Among the main ones are glucocorticoids, potassium bromide, phenobarbital, sulfonamides, propranolol, and anti-inflammatory drugs such as carprofen, meloxicam, and ketoprofen—each acting through a different mechanism, such as suppression of the HHT axis or displacement of binding to carrier proteins.
What is the difference between drug-induced interference and sick euthyroid syndrome?
In drug-induced interference, the drug itself alters thyroid hormones. In sick-euthyroid syndrome, it is a systemic disease (renal, hepatic, neoplastic, infectious) that reduces total T4 as the body’s response to the underlying disease; it is not a primary thyroid problem.
How can you avoid a misdiagnosis of hypothyroidism caused by medications?
Ideally, you should list all medications currently being taken before requesting the test, consider the timing of the sample collection in relation to when the medication was started or discontinued, interpret the results in conjunction with the patient’s overall clinical picture, and repeat the test after the condition has resolved or the medication has been adjusted if there is any uncertainty.