Hypothyroidism (Canine) — NAVLE Prep

Quick anchor

Trigger

Weight gain + low energy + symmetric alopecia

First step

Test the right dog, not every low T4

Confirm

Free T4 + cTSH in context

Trap

Low total T4 alone is not diagnostic

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

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

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.

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

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

Practice questions

Pre-built NAVLE-style · Hypothyroidism

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Q1Pattern recognition

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?

Q2Trap interpretation

A hospitalized dog with pneumonia is receiving glucocorticoids. A screening panel shows a low total T4. Which conclusion is most appropriate?

Q3Next best step

A stable outpatient dog has lethargy, weight gain, bilateral alopecia, and fasting hypercholesterolemia. Which diagnostic plan is most appropriate?

Q4Monitoring

A dog with confirmed hypothyroidism has been started on levothyroxine. Which follow-up plan is best?

Q5Trap question

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?