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 6

Practice Qs

Traps

High

Exam freq.

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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

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

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

Mini cases — apply the decision framework

Pattern recognition

7yr FS Golden Retriever is lethargic, has gained weight, and has a dull haircoat with symmetric truncal alopecia. Cholesterol is elevated. What diagnosis should move up your list?

Interpretation

10yr dog with pneumonia has a low total T4 on a chemistry add-on panel. The dog has no compatible chronic skin or metabolic history. What is the safest interpretation?

Monitoring

A dog with confirmed hypothyroidism has improved energy after levothyroxine, but the coat has only minimal regrowth after three weeks. Should you abandon the diagnosis?

Clinical application tools

These help with baseline interpretation and dose checks. They do not replace choosing the right patient for testing.

Normal values Dose calculator Unit converter

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?

AHyperadrenocorticism

BHypothyroidism

CAcute pancreatitis

DProtein-losing nephropathy

EPrimary hypoadrenocorticism

Q2Trap interpretation

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

AThe dog definitely has primary hypothyroidism

BLevothyroxine should be started immediately

CThe result may reflect illness or drug effect rather than true thyroid failure

DcTSH will always be normal in euthyroid sick syndrome

EHypercholesterolemia must be present to interpret the result

Q3Next best step

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

ADiagnose hypothyroidism from low total T4 alone

BConfirm with thyroid testing interpreted in clinical context

CSkip testing and treat empirically with levothyroxine

DPerform ACTH stimulation testing first

ERepeat testing only if the dog becomes critically ill

Q4Monitoring

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

ANo recheck is needed if the dog seems brighter at home

BCheck total T4 immediately before the morning pill in 48 hours

CRepeat cTSH weekly until normal

DRecheck after several weeks and sample 4–6 hours after dosing

EMonitor only cholesterol because thyroid values fluctuate too much

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?

APrimary allergic skin disease is more likely than hypothyroidism

BHypothyroidism is confirmed because total T4 is low

CThe dog likely has Addison's disease instead

DLevothyroxine should be started as a diagnostic trial

EcTSH is unnecessary because pruritus is specific for hypothyroidism