Hypothyroidism is a prevalent endocrine disorder in dogs, characterized by the thyroid glands’ insufficient production of thyroxine (T4) and triiodothyronine (T3) hormones. The most frequently diagnosed form is primary hypothyroidism, stemming from either idiopathic thyroid gland atrophy or immune-mediated lymphocytic thyroiditis. Less common origins include congenital conditions arising from thyroid hormone dyshormonogenesis, irregular thyroid-stimulating hormone (TSH) production, or atypical thyroid gland development.
Thyroid hormones play a critical role in numerous metabolic functions, and diminished levels can lead to a range of clinical signs and laboratory abnormalities indicative of hypothyroidism. Accurate Canine Hypothyroidism Diagnosis necessitates a series of hormone tests and should never rely solely on a low T4 concentration.
Understanding the Clinical Presentation of Canine Hypothyroidism
Hypothyroidism commonly manifests in middle-aged dogs, although it can affect dogs of any age and breed. The clinical signs, detailed in BOX 1, can be vague, potentially leading to overdiagnosis. Lethargy and weight gain are frequently observed symptoms. These signs may develop gradually and might initially go unnoticed by pet owners. A hallmark of canine hypothyroidism is often changes in the hair coat, such as bilaterally symmetrical, non-itchy hair loss on the trunk or areas of friction, alopecia following clipping, and a lackluster coat, as illustrated in FIGURE 1. Skin changes may also occur, including scaling, seborrhea, hyperpigmentation, and recurring skin or ear infections (pyoderma or otitis externa).
FIGURE 1. Dog presenting with clinical signs of hypothyroidism, notable for excessive body weight, a dull and dry hair coat, and skin scaling. These are common dermatological indicators that can assist in canine hypothyroidism diagnosis.
In rarer instances, canine hypothyroidism can be associated with conditions like megaesophagus, vestibular dysfunction, facial nerve paralysis, and atherosclerosis.
BOX 1 Common Clinical Signs Associated with Canine Hypothyroidism
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Metabolic Rate Reduction Signs:
- Lethargy or reduced mental alertness
- Reduced activity or reluctance to exercise
- Unexplained weight gain
- Sensitivity to cold or seeking warmth
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Dermatologic Changes:
- Symmetrical, non-pruritic (non-itchy) hair loss
- Alopecia post-clipping
- Dry and brittle hair coat
- Skin scaling
- Hyperpigmentation (darkening of the skin)
- Recurrent pyoderma (bacterial skin infection) or otitis externa (ear infection)
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Uncommon Signs and Syndromes:
- Incoordination or ataxia
- Ocular Changes:
- Lipid deposits in the cornea
- Peripheral Nervous System Issues:
- Facial nerve paralysis
- Laryngeal paralysis
- Polyneuropathy (nerve damage)
- Vestibular Dysfunction (balance problems)
- Megaesophagus or esophageal motility issues
- Cardiovascular Abnormalities:
- Bradycardia (slow heart rate)
- Worsening of pre-existing cardiac conditions
- Atherosclerosis (artery hardening)
- Reproductive Effects:
- Increased perinatal mortality in puppies
- Lower birth weights of puppies
- Myxedema Coma (severe hypothyroidism crisis):
- Depressed consciousness
- Thermoregulation problems
- Bradycardia
- Hypoventilation
- Thickened skin
Step-by-Step Diagnostic Process for Canine Hypothyroidism
Canine hypothyroidism diagnosis should only be pursued when there is a strong clinical suspicion based on the dog’s medical history and physical examination findings, particularly those listed in BOX 1. Results from a complete blood count and serum biochemistry panel can increase suspicion. Misdiagnosis can occur if testing is initiated solely due to a dog being overweight or because a T4 measurement is included in a routine biochemistry panel.
A structured approach is crucial for accurately diagnosing canine hypothyroidism, as outlined in FIGURE 2.
FIGURE 2. Diagnostic algorithm for canine hypothyroidism diagnosis, illustrating a stepwise approach from initial bloodwork to confirmatory thyroid hormone tests.
Step 1: Initial Screening with Minimum Database for Supportive Evidence
A complete blood count, serum biochemistry panel, and urinalysis are valuable in excluding other conditions that might affect thyroid test results. While abnormalities found in these tests are not specific to hypothyroidism, they can support clinical suspicion.
Elevated cholesterol levels are seen in approximately 75% of dogs with hypothyroidism. While mild hypercholesterolemia alone isn’t a reason to test for hypothyroidism, it strengthens the suspicion. Liver enzymes may also be mildly elevated. A mild, non-regenerative anemia is present in about 30% to 40% of hypothyroid dogs. Urinalysis is typically unremarkable. If dilute urine is present, further investigation for concurrent illnesses or other causes of clinical signs is warranted.
Step 2: Thyroid Screening with Total T4 Concentration Test
The total T4 concentration test is a useful initial screening tool for canine hypothyroidism diagnosis. Its sensitivity is reported to be high, ranging from 89% to 100%. If the T4 concentration falls well within the normal reference range, hypothyroidism is highly unlikely, and further thyroid testing is generally unnecessary. Free T4 (fT4) and thyroid-stimulating hormone (TSH) tests are only indicated if the total T4 concentration is low, as shown in FIGURE 2. Performing combined T4, fT4, and TSH testing upfront is not recommended at this stage and can incur unnecessary costs, as a normal T4 level effectively rules out hypothyroidism in most cases.
However, a low T4 concentration is not definitively diagnostic of hypothyroidism. Besides natural daily fluctuations, several medications can lower serum T4 levels in dogs (BOX 2), and some can also affect fT4 and TSH concentrations. Certain drugs, like trimethoprim-sulfamethoxazole, can directly impact the pituitary-thyroid axis, potentially leading to drug-induced hypothyroidism. Furthermore, non-thyroidal illnesses can disrupt thyroid hormone metabolism, resulting in euthyroid sick syndrome (BOX 3). While fT4 concentrations are less susceptible to concurrent illness, severe illness can still depress fT4 levels. Therefore, thyroid testing should ideally be avoided in systemically ill dogs. If testing is performed in a sick dog, results must be interpreted cautiously.
BOX 2 Medications Known to Alter Canine Thyroid Hormone Function or Test Results
- High-dose Prednisone
- Phenobarbital
- Trimethoprim-sulfamethoxazole
- High-dose Aspirin
- Clomipramine
- Thyroxine supplementation
BOX 3 Euthyroid Sick Syndrome Explained
Euthyroid sick syndrome describes a condition where non-thyroidal illnesses suppress circulating thyroid hormone concentrations. The underlying mechanism is complex and involves alterations in hormone distribution, metabolism, and binding to proteins.
Given that canine hypothyroidism diagnosis is not typically an emergency, sending samples to a reference laboratory for testing is advisable. As further confirmatory tests are often required, collecting and storing extra serum when initially drawing blood for the T4 test is a practical step.
It’s important to note that standard T4 reference ranges may not be applicable to sighthound breeds, as healthy sighthounds naturally have lower T4 concentrations compared to other breeds.
Step 3: Confirmation with Free T4 (fT4) and/or TSH Concentration Tests
When a dog suspected of hypothyroidism shows a low total T4 concentration, evaluating fT4 and/or TSH concentrations is crucial to confirm or rule out the diagnosis, as outlined in TABLE 1. Elevated TSH levels, in conjunction with low T4, strongly support a diagnosis of hypothyroidism. However, a significant portion (13% to 38%) of hypothyroid dogs may have normal TSH concentrations. Therefore, a normal TSH level does not definitively exclude hypothyroidism. Due to this limitation, simultaneously evaluating both fT4 and TSH as confirmatory tests is often beneficial. A low fT4 concentration, especially when coupled with clinical signs and low T4, is highly indicative of hypothyroidism.
If T4 is low but fT4 is within the normal reference range, canine hypothyroidism diagnosis should be reconsidered, and other potential causes for the dog’s clinical signs should be explored. T3 concentrations are unreliable for diagnosis due to wide variations and are not diagnostically useful.
Canine Hypothyroidism Treatment and Management
Initiating Levothyroxine Therapy
Studies indicate that most dogs with hypothyroidism respond well to once-daily levothyroxine administration, typically starting at a dosage of 0.02 mg/kg orally every 24 hours. Some veterinarians may initially prescribe twice-daily levothyroxine (0.02 mg/kg orally every 12 hours) and attempt to reduce to once-daily dosing once clinical signs are well-managed. Improvement in lethargy is often observed within a few weeks. Most clinical signs, including dermatological changes, show improvement within 4 to 6 weeks, although full resolution of skin and coat issues may take several months.
Monitoring and Adjusting Levothyroxine Dosage
Approximately 4 weeks after initiating or adjusting levothyroxine therapy, blood should be drawn 4 to 6 hours post-pill for T4 measurement. (While T4 levels can be assessed as early as 2 weeks post-therapy change, waiting 4 weeks allows for concurrent evaluation of clinical sign improvement). The target post-pill T4 concentration should be in the upper portion of the reference range or slightly above.
If the post-pill T4 concentration is below the target range, the levothyroxine dose should be increased by 25%. The T4 concentration should then be re-evaluated in 2 to 4 weeks. This dose adjustment process is repeated until the post-pill T4 concentration reaches the target range. Conversely, if the post-pill T4 concentration is too high, the dose should be reduced by 25% and re-checked. Once a stable and effective dose is established, the interval between monitoring visits can be extended to every 6 months.
Addressing Treatment Failure and Potential Adverse Effects
Treatment failure in canine hypothyroidism is uncommon. Potential reasons for not achieving the target T4 concentration include owner non-compliance with medication administration or the dog refusing medication. Variable gastrointestinal absorption of levothyroxine is also a possible factor. If the target post-pill T4 concentration is reached, but clinical signs are not adequately controlled, increasing the dosing frequency to twice daily should be considered. Furthermore, re-evaluating the initial canine hypothyroidism diagnosis is warranted. If the diagnosis is confirmed and T4 levels are well-managed, consider investigating for concurrent disorders that might be contributing to the persistent clinical signs.
Dogs generally exhibit a high tolerance to levothyroxine supplementation. However, excessive doses may lead to signs such as polyuria/polydipsia (increased urination and thirst) and hyperactivity.
Conclusion: Ensuring Accurate Canine Hypothyroidism Diagnosis
Thyroid testing should be reserved for patients with a genuine clinical suspicion of thyroid disease. While measuring total T4 concentration is a helpful initial step to rule out hypothyroidism, it is insufficient for definitive diagnosis. Comprehensive testing, including serum T4 concentration along with fT4 and/or TSH levels, is essential for accurate canine hypothyroidism diagnosis and minimizes the risk of misdiagnosis. Given the need for lifelong therapy, confirming the diagnosis definitively before commencing treatment is crucial for the well-being of the dog.
