About This Page
This is a clinician-written, evidence-based guide aligned to the MCC Examination Objectives. It is structured by clinical presentation — the way the MCCQE tests and the way patients actually present. Management reflects current Canadian guidelines (CMA, CFPC, CPS). Always cross-reference with institutional protocols and clinical judgment.
The Bottom Line
- TSH is the best screening test for primary hypothyroidism; free T4 distinguishes overt from subclinical disease
- Hashimoto thyroiditis is the most common cause in iodine-sufficient settings
- Myxoedema coma is rare but life-threatening: hypothermia, bradycardia, hypotension, hypoventilation, hyponatremia, and altered mental status
- Levothyroxine is taken consistently on an empty stomach; dose cautiously in older adults and coronary disease
- Subclinical hypothyroidism is not automatically treated — consider TSH level, symptoms, pregnancy, anti-TPO, goitre, and cardiovascular context
Approach to the Presentation
Hypothyroidism is a common primary care presentation but the symptoms are non-specific. The MCCQE1 approach starts by confirming biochemical thyroid dysfunction rather than treating fatigue alone. Primary hypothyroidism shows high TSH with low free T4 when overt; central hypothyroidism shows low or inappropriately normal TSH with low free T4 and requires pituitary evaluation. In Canada, management is usually levothyroxine replacement with TSH monitoring, attention to drug interactions, pregnancy status, cardiac risk, and avoidance of unnecessary thyroid imaging unless there is palpable thyroid enlargement or nodularity.
Differential Diagnosis
| diagnosis | likelihood | key features | distinguishing test |
|---|---|---|---|
| Myxoedema Coma | must-not-miss | Altered mental status, hypothermia, bradycardia, hypotension, hypoventilation, hyponatremia, precipitated by infection, cold exposure, sedatives, MI, or stopping levothyroxine | Clinical emergency; high TSH/low free T4 in primary disease; do not delay IV thyroid hormone and hydrocortisone with ICU support |
| Central Hypothyroidism / Hypopituitarism | must-not-miss | Fatigue, amenorrhoea, low libido, headaches, visual field symptoms, postpartum haemorrhage history, other pituitary hormone deficits | Low free T4 with low/inappropriately normal TSH; pituitary hormone panel and MRI |
| Hashimoto Thyroiditis | common | Gradual fatigue, weight gain, cold intolerance, constipation, goitre, family autoimmune disease | High TSH, low/normal free T4, positive anti-TPO antibodies |
| Iatrogenic Hypothyroidism | common | History of thyroidectomy, radioactive iodine, neck radiation, immune checkpoint inhibitor, lithium, amiodarone | Medication/procedure history plus thyroid function tests |
| Subclinical Hypothyroidism | common | Often asymptomatic or mild symptoms; high TSH with normal free T4 | Repeat TSH/free T4; anti-TPO helps progression risk |
| Depression / Sleep Disorder / Anaemia Mimic | common | Fatigue, low mood, weight change, cognitive slowing without biochemical hypothyroidism | Normal TSH/free T4; evaluate CBC, ferritin, sleep, mood, medications |
| Drug or Absorption-related Undertreatment | common | Persistent hypothyroid labs despite prescription; calcium, iron, PPIs, cholestyramine, celiac disease, missed doses | Medication timing review; TSH remains elevated |
| Postpartum Thyroiditis | less common | Within first year postpartum; transient hyperthyroid phase followed by hypothyroid phase; often anti-TPO positive | Thyroid function pattern over time; low uptake during thyrotoxic phase |
| Congenital or Paediatric Hypothyroidism | rare | Growth failure, developmental delay, prolonged jaundice, constipation, large fontanelle in infant | Newborn screen or TSH/free T4 depending on age |
Red Flags & Key History
Symptoms
Altered mental status, hypothermia, dyspnea, severe weakness, or collapse — myxoedema coma
Headache, visual field loss, amenorrhoea, hypotension, or postpartum haemorrhage history — central/pituitary cause
Pregnancy or planning pregnancy with hypothyroidism — urgent dose and TSH target considerations
Chest pain or known coronary disease before thyroid replacement escalation
Neck mass, rapid thyroid growth, hoarseness, or lymphadenopathy
Fatigue, weight gain, cold intolerance, constipation, dry skin, hair loss, heavy menses
Depression, cognitive slowing, carpal tunnel symptoms, infertility, hyperlipidemia
Signs
Hypothermia, bradycardia, hypotension, hypoventilation, delayed reflexes
Periorbital puffiness, dry coarse skin, macroglossia, hoarse voice
Goitre suggesting Hashimoto thyroiditis or iodine/drug-related disease
Galactorrhoea, visual field defects, or other pituitary signs
Effusions, non-pitting oedema, or heart failure features in severe disease
Approach to Investigation
First-line
TSHBest screening test for primary hypothyroidism
Free T4Distinguishes overt hypothyroidism from subclinical hypothyroidism and identifies central hypothyroidism
Anti-TPO antibodiesSupports autoimmune thyroiditis and predicts progression in subclinical disease
CBC, ferritin, B12, lipids, creatinine, pregnancy test when indicatedAssess mimics, associated autoimmune disease, cardiovascular risk, and reproductive context
Second-line
Pituitary hormone panelIf low free T4 with low or normal TSH, or symptoms/signs of pituitary disease
Morning cortisol before levothyroxine if central disease suspectedTreat adrenal insufficiency before thyroid hormone to avoid adrenal crisis
Repeat thyroid function testsConfirm persistent abnormality before treating mild subclinical disease unless pregnant or strongly indicated
Specialist
Pituitary MRIFor central hypothyroidism, visual symptoms, headaches, or multiple pituitary deficits
Endocrinology referralCentral hypothyroidism, pregnancy complexity, myxoedema coma, difficult titration, malabsorption, or unusual paediatric presentations
Management Principles
Canadian primary care practice informed by CMAJ thyroid reviews and Choosing Wisely Canada endocrine recommendations1
Routine overt primary hypothyroidism
- Start levothyroxine and adjust based on TSH every 6-8 weeks until stable
- Use lower starting doses in older adults or coronary artery disease; titrate gradually
- Counsel to take consistently on an empty stomach and separate from calcium, iron, and bile-acid binders
2
Subclinical hypothyroidism
- Repeat testing to confirm persistence
- Consider treatment if TSH persistently >10 mIU/L, pregnancy/planning pregnancy, goitre, anti-TPO positivity, significant symptoms, or selected cardiovascular risk contexts
- Avoid reflex treatment of non-specific symptoms with normal free T4 and only mildly elevated TSH without shared decision-making
3
Myxoedema coma
- ICU care, airway/ventilation support, passive rewarming, IV levothyroxine, stress-dose hydrocortisone, correction of hyponatremia/hypoglycemia, and treatment of precipitant
Complications & Pitfalls
- Treating fatigue without confirming disease: Symptoms are non-specific; use TSH and free T4
- Missing central hypothyroidism: TSH may be normal; low free T4 with pituitary features is the clue
- Giving levothyroxine before steroid coverage in hypopituitarism: This can precipitate adrenal crisis
- Over-replacement: Excess levothyroxine increases AF and fracture risk, especially in older adults
- Unnecessary ultrasound: Abnormal thyroid labs alone do not require ultrasound without palpable abnormality
MCCQE1 Exam Tips
- 1Primary hypothyroidism = high TSH. Central hypothyroidism = low free T4 with low/inappropriately normal TSH
- 2Myxoedema coma is a clinical emergency: hypothermia, bradycardia, hypotension, hypoventilation, altered mental status
- 3Levothyroxine dose changes are assessed after 6-8 weeks, not after a few days
- 4Check medication timing if TSH stays high: calcium, iron, PPIs, and missed doses are common
- 5Do not order thyroid ultrasound for hypothyroidism unless there is a palpable nodule/goitre
- 6In suspected pituitary disease, check cortisol before starting thyroid hormone
- 7Subclinical hypothyroidism requires context; not every mildly high TSH needs treatment
practicetest your knowledge on hypothyroidismApply what you've learnt with MCCQE1-style questions from the iatroX Q-Bank — endocrine & metabolic and beyond.
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