Tier 1 — must know
Canine
Endocrine
High yield
Hypothyroidism
Primary hypothyroidism · dermatology + internal medicine overlap · classic outpatient diagnosis
⏱ 2–3 min read · Topic 2 of 141
5
Practice Qs
4
Traps
High
Exam freq.
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Your status
Study step
High-yield takeaways
- Recognize the classic presentation, then narrow the case using signalment, timeline, exam findings, diagnostics, and response to treatment.
- Use the decision framework, traps, differentials, and related questions to rehearse NAVLE-style next-best-step reasoning.
- This educational study page is not a clinical protocol; confirm patient-specific decisions with current references and clinician judgment.
30-second revision
Classic patternLethargy + weight gain + symmetric alopecia
Key labsFree T4↓ cTSH↑ cholesterol↑
Diagnosis ruleLow total T4 alone is not enough
Best confirmationInterpret free T4 + cTSH in context
TreatmentLevothyroxine
MonitoringPost-pill recheck at 4–8 weeks
Critical trapSick dog low T4 ≠ hypothyroidism
Exam core — read this first
Classic patient → lethargy, weight gain, bilateral alopecia, recurrent skin infections
Low total T4 alone → never enough to diagnose hypothyroidism
Best confirmation → low free T4 with high cTSH in the right clinical context
Treatment → levothyroxine with post-pill monitoring
Clinical mechanism — only what matters
↓ Thyroid hormone → lower metabolic rate → lethargy, exercise intolerance, weight gain
↓ Skin and hair turnover → symmetric alopecia, seborrhea, recurrent pyoderma
↓ Neuromuscular drive → weakness, facial droop, occasional bradycardia
The board question is usually not about thyroid physiology depth. It is about choosing the right patient and interpreting tests correctly.
Pattern recognition
Core pattern
Lethargy + mental dullness
Weight gain
Symmetric alopecia
Supporting clues
Hypercholesterolemia
Seborrhea / pyoderma
Chronic otitis
Heat-seeking behavior
Middle-aged medium-large breed dog
NAVLE trigger: Hypothyroidism is a slow outpatient diagnosis. The common trap is overcalling it in a sick dog with a low screening T4.
Decision core — what NAVLE actually asks
Classic stable outpatient — skin + metabolic pattern fits
→ Run routine database, then confirm with a thyroid panel interpreted in context
Hospitalized or systemically ill dog with low total T4
→ Do not diagnose hypothyroidism from that result alone; consider euthyroid sick syndrome or drug effects first
Pruritus or otitis drives the visit
→ Work up allergic skin disease or infection first; hypothyroidism is usually a non-pruritic metabolic pattern
Confirmed primary hypothyroidism
→ Start levothyroxine and recheck a post-pill thyroid value after several weeks
Key interpretation
Total T4
↓ Low
Useful screen only; not specific
Free T4
↓ Low
Better support for true disease
cTSH
↑ High
Supports primary hypothyroidism
Cholesterol
↑ High
Classic supportive clue
CBC
Mild nonregenerative anemia
Common but not specific
Drugs / illness
Can lower T4
Steroids and illness can mislead you
⚠ A low total T4 does not automatically equal hypothyroidism. NAVLE likes the sick-dog low T4 trap.
Treatment
Start
Levothyroxine orally
Dose consistently so interpretation of monitoring is meaningful.
Monitor
Recheck thyroid values 4–6 hours post-pill after 4–8 weeks
Match the number to clinical improvement, not the number alone.
Also
Treat secondary pyoderma / otitis if present
Energy improves earlier than haircoat. Owners should expect that sequence.
NAVLE traps — where students lose marks
Low total T4 alone is not a diagnosis
Illness and drugs can suppress total T4. Boards test whether you know when the screen is misleading.
Do not call every alopecic dog hypothyroid
A very itchy dog with recurrent otitis is more often allergic than hypothyroid.
Obesity alone is not hypothyroidism
The exam pattern needs metabolic plus dermatologic or neuromuscular clues.
Clinical improvement happens in stages
Coat improvement lags behind energy improvement. That is expected early in therapy.
Do not ignore concurrent illness
Systemic disease can suppress thyroid values and should be controlled before diagnosis closure.
Do not monitor by lab values alone
Dose interpretation should match timing after the pill and clinical response.
Differentials — how to separate these on NAVLE
Fast separator: True hypothyroidism is a chronic, non-pruritic metabolic + dermatologic syndrome. The exam often contrasts it with sick-dog low T4 and other endocrine look-alikes.
| Disease | T4 pattern | Skin pattern | Key separator |
|---|---|---|---|
| Hypothyroidism | Low free T4, often high TSH | Non-pruritic symmetric alopecia | Weight gain + lethargy + hypercholesterolemia |
| Euthyroid sick syndrome | Low total T4 | Variable | Sick patient; thyroid drop is secondary |
| Hyperadrenocorticism | Usually normal thyroid values | Thin skin, calcinosis | Panting, PU/PD, polyphagia |
| Chronic allergy / pyoderma | Normal | Usually pruritic | Itch drives the case |
| Obesity / deconditioning | Normal | Normal | No classic skin or lab pattern |
Clinical application tools
These help with baseline interpretation and dose checks. They do not replace choosing the right patient for testing.
Related questions
Pre-built NAVLE-style · Hypothyroidism
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A 7yr FS Golden Retriever presents for chronic lethargy, weight gain, and recurrent superficial pyoderma. Examination reveals bilaterally symmetric truncal alopecia. Which diagnosis best fits this pattern?
A hospitalized dog with pneumonia is receiving glucocorticoids. A screening panel shows a low total T4. Which conclusion is most appropriate?
A stable outpatient dog has lethargy, weight gain, bilateral alopecia, and fasting hypercholesterolemia. Which diagnostic plan is most appropriate?
A dog with confirmed hypothyroidism has been started on levothyroxine. Which follow-up plan is best?
A dog with severe pruritus, recurrent otitis, and self-trauma has a borderline-low total T4 but normal energy and body weight. Which explanation is most likely?