The short answer
Hypothyroidism in dogs is a deficiency of thyroid hormone, most often arising in adults from destruction of the thyroid gland itself, with broad, nonspecific signs such as lethargy, weight gain and coat changes. The cited sources describe diagnosis as a combination of compatible clinical signs, supportive routine bloodwork and thyroid function tests (total T4, free T4 and TSH), each of which has known false results, and illness, breed and many drugs can lower thyroid values in dogs whose thyroid glands are normal. Treatment is lifelong oral levothyroxine, monitored by clinical response and post-pill total T4. Several sources say the disease is overdiagnosed, and one retrospective study of 102 dogs found that reviewing specialists judged treatment not indicated in about half of the dogs already receiving levothyroxine.
Key takeaways
- The Merck Veterinary Manual reports that more than 95% of clinical canine cases appear to be primary hypothyroidism, mainly from lymphocytic thyroiditis or idiopathic thyroid atrophy [1].
- Merck describes total T4 as a good first screening test with sensitivity of roughly 90%, but says a low T4 by itself cannot establish the diagnosis [1].
- According to a Canadian Veterinary Journal review, low total T4 and free T4 together with a high TSH carry specificity above 95% [2].
- A 2023 JVIM review says glucocorticoids, phenobarbital, some NSAIDs, sulfonamides, inhalant anesthetics, clomipramine, toceranib, amiodarone and trilostane all warrant cautious interpretation of thyroid results [4].
- In a retrospective study of 102 treated dogs, three internal medicine diplomates reached a consensus that levothyroxine was not indicated in 52% of cases [3].
- Merck states that improvements in lethargy and mentation occur within 2 weeks of treatment, while skin and weight changes may need up to 3 months [1].
Doses on this pageEvery dose below is quoted from the cited source. Verify against the current label/formulary and the patient's full clinical picture.
Causes and who is affected
Merck identifies lymphocytic thyroiditis and idiopathic thyroid atrophy as the two most common causes of adult-onset primary disease [1]. Thyroiditis is described as likely immune-mediated, and the CVJ review notes that idiopathic atrophy may represent the end stage of thyroiditis [1][2]. A JVIM study cites these two causes as each accounting for roughly half of diagnosed cases [3].
According to Merck, secondary hypothyroidism, from loss of pituitary TSH, is uncommon and usually linked to a pituitary tumor, and congenital forms are rare [1]. Merck reports the disease is most common in dogs 4 to 10 years old, with no apparent sex predilection, and usually in mid- to large-size breeds [1]. The CVJ review gives a mean onset of 7 years and says spayed and neutered dogs may carry higher risk than intact dogs [2]. The JVIM study authors cite studies placing prevalence at approximately 1% of the canine population [3].
Clinical signs
Merck links many signs to slowed cellular metabolism: mental dullness, lethargy, exercise intolerance and weight gain without increased appetite, with heat seeking or hypothermia in some dogs [1]. The CVJ review notes that obesity is present in only about half of cases [2].
Skin and coat changes are common. Merck describes nonpruritic, usually bilaterally symmetric hair thinning or alopecia in about two-thirds of affected dogs, often beginning over points of wear [1]. The CVJ review adds:
- seborrhea
- hyperpigmentation
- ceruminous otitis
- a dull and brittle coat
- bacterial pyoderma in 10 to 23% of dogs
The same review says pruritus is not a hallmark unless infection is present [2]. Thickened facial skin from dermal mucin produces what both sources call a "tragic facial expression" [1][2].
Other reported signs include reproductive disturbances in intact dogs and neurological disorders such as megaesophagus, laryngeal paralysis, facial nerve paralysis and vestibular disease [1]. Myxedema coma is described as a rare extreme form with stupor, hypoventilation, hypotension, bradycardia and profound hypothermia [1][2].
Thyroid function tests and interpretation
None of the cited sources describes a formal staging system. The sources classify the disease by origin (primary, secondary, congenital) rather than by stage [1][2].
Routine bloodwork
Merck reports hypercholesterolemia in approximately 80% of hypothyroid dogs and a normocytic, normochromic, nonregenerative anemia in 40 to 50% [1]. The CVJ review gives lower figures, about 75% and 25 to 40% respectively [2]. Merck describes these changes as nonspecific but supportive evidence in a dog with relevant clinical signs [1].
Hormone tests
| Test | What the sources report |
|---|
| Total T4 | Sensitivity about 90%, low specificity; a value well within the reference interval suggests normal function [1]. CVJ says disease can be ruled out if T4 sits in the upper 50% of the range [2]. |
| Free T4 by equilibrium dialysis | Less affected by nonthyroidal illness and unaffected by T4 autoantibodies [1][2]. Bolton and Panciera call it the only accurate free T4 method in dogs [4]. |
| TSH | Within the reference range in 20 to 40% of confirmed cases per Merck [1]; about 25% per the JVIM drug review [4]. Falsely high in some euthyroid dogs with other illness [1]. |
| Total T3 | Not usually measured, because values in hypothyroid dogs vary [1][2]. |
The JVIM study cites published sensitivities of 89 to 100% for total T4, 58 to 87% for TSH and 80 to 98% for free T4, with specificities of 73 to 82%, 82 to 100% and 78 to 94% [3]. Merck says single-stage analogue free T4 assays do not appear better than total T4 [1].
Merck warns that TSH results "should never be evaluated alone" [1]. Thyroglobulin autoantibodies indicate thyroiditis but give no information on thyroid function. Merck notes that only about one-third of dogs with high levels go on to develop hypothyroidism [1]. Antibodies against T4 or T3 can falsely raise measured hormone values and mask the disease [1][2].
Merck describes the TSH stimulation test as accurate but limited by TSH availability, with a recommended human recombinant TSH dose of "75 mcg, IV, with collection of 0- and 6-hour samples" [1]. Merck also describes quantitative technetium-99m thyroid scintigraphy as showing little to no overlap between primary hypothyroidism and nonthyroidal illness [1].
Factors that distort results
According to Merck, nonthyroidal illness lowers total T4 and T3 in proportion to severity, and testing should be postponed until such illness resolves where possible [1]. The CVJ review applies the same caution to diabetes mellitus and hyperadrenocorticism [2]. Greyhounds and other sighthounds have lower normal T4, and Merck notes T4 declines with age [1][2].
Bolton and Panciera reviewed drug effects in dogs and gave withdrawal intervals where data allowed [4]:
| Drug | Reported effect and timing |
|---|
| Glucocorticoids | Immunosuppressive doses lower TT4, TT3 and fT4; at least 1 week off after 3 weeks of treatment is likely to let T4 values return to baseline [4] |
| Phenobarbital | Long-term use lowers TT4 and often fT4; the authors recommend waiting at least 6 weeks after stopping [4] |
| Potentiated sulfonamides | Can produce results indicative of primary hypothyroidism; testing at least 4 weeks after stopping [4] |
| Aspirin | Can drop TT4 below range within 24 hours; withdrawal of at least 7 to 14 days suggested [4] |
| Inhalant anesthetics | Testing should be avoided for 14 days after anesthesia or surgery [4] |
| Clomipramine, amiodarone | Effects documented, but no withdrawal interval can be recommended [4] |
The authors note that potassium bromide and imepitoin did not alter thyroid tests in the studies reviewed [4].
Overdiagnosis in first-opinion practice
Travail and colleagues retrospectively reviewed 102 dogs treated with levothyroxine at seven UK first-opinion practices between 2016 and 2021 [3]. Three internal medicine diplomates independently judged each case, and by majority they considered treatment not indicated in 53 of 102 dogs [3]. The most common reason cited was insufficient clinical and clinicopathologic evidence, with concurrent illness and interfering drugs also commonly listed [3]. A higher share of dogs whose T4 was measured on an in-house analyzer were judged not to need treatment, and this association was statistically significant for two of the three reviewers [3].
The authors acknowledge limits: the design was retrospective, some clinical notes may have been missing, and inter-rater agreement was poor to moderate [3]. They concluded that thyroid testing should be reserved for dogs with "a high pretest probability of the disease" and disagreed with routine T4 screening of senior dogs [3]. They also wrote that misdiagnosis can delay recognition of other disease [3].
Treatment and monitoring
Merck describes levothyroxine as the replacement compound of choice. It states that an FDA-approved product is given at "22 mcg/kg, PO on an empty stomach, every 24 hours [or divided in two daily doses], lifelong" [1]. The CVJ review gives "0.02 mg/kg BW given once or twice daily, up to a maximum of 0.8 mg twice daily" and calls the once versus twice daily choice controversial [2]. That review recommends dosing 2 to 3 hours before food [2]. Merck notes that canine doses are substantially higher than human doses, a point to communicate to pharmacists [1].
Merck names clinical improvement as the most important indicator of success. It describes T4 monitoring, generally after 4 to 8 weeks of supplementation, when clinical improvement is marginal or clinical signs of thyrotoxicosis are present [1]. With once-daily dosing, Merck says the 4 to 6 hour post-pill T4 should sit at the high end of or slightly above the reference interval [1]. After stabilization, Merck suggests checks 1 to 2 times per year, while the CVJ author describes monitoring every 4 to 6 months [1][2]. Merck lists polyuria, polydipsia, weight loss, panting, nervousness and tachycardia as signs of thyrotoxicosis, which it calls rare [1].
Merck says the most common cause of treatment failure is an incorrect diagnosis [1]. The CVJ review suggests considering another diagnosis or concurrent disease if signs have not improved within 8 weeks despite target post-pill T4 [2].
Prognosis and client communication
The CVJ review describes the prognosis for complete recovery as excellent in most cases and guarded for myxedema coma. It notes that congenital cases may retain mental dullness and skeletal abnormalities [2]. Some neurological deficits may persist [1][2]. Practical points raised in the sources include lifelong daily medication, periodic blood sampling, and consistent timing relative to food on monitoring days [1][2][3]. The CVJ author tells owners to expect more shedding and dandruff around 3 to 4 weeks after starting treatment as old hairs are replaced. The review also notes that hair regrowth can take several months [2].
