Hypothyroidism

💡 Always start with a lower dose of levothyroxine in the elderly and in those with angina or heart failure, to avoid precipitating arrhythmia or ischaemia.

📖 Introduction

• Hypothyroidism = inadequate thyroxine production for normal metabolism.
• Thyroid produces both T4 and T3, but T3 is the active form at tissue level.
• TSH is the best single test: ↑TSH is a highly sensitive marker of primary hypothyroidism.
• A normal TSH in a treated patient usually indicates adequate replacement and euthyroidism.

⚙️ Aetiology

• Primary hypothyroidism (95%): intrinsic thyroid failure (iodine deficiency, autoimmune thyroiditis, post-surgical, post-radioiodine).
• Secondary hypothyroidism: pituitary or hypothalamic disease → insufficient TSH secretion (TSH low/normal despite low T4).

🧬 Causes of Primary Hypothyroidism

• Iodine deficiency: still the most common cause worldwide.
• Autoimmune: Hashimoto’s thyroiditis (± goitre) or atrophic thyroiditis (no goitre). Often associated with other autoimmune disease.
• Post-ablative: surgery, radioiodine, external radiotherapy.
• Drugs: carbimazole, propylthiouracil, lithium, amiodarone, interferons, thalidomide, rifampicin, excess iodine (e.g. kelp supplements).
• Transient thyroiditis: subacute (de Quervain’s), often post-viral, painful goitre.
• Postpartum thyroiditis: transient hyper → hypo → recovery within 1 year.
• Infiltrative disease: amyloidosis, sarcoidosis, haemochromatosis, TB, scleroderma.
• Congenital: agenesis, hypoplasia, ectopic gland, or enzyme defects (causes cretinism if untreated).

🧑‍⚕️ Clinical Features

• General: tiredness, lethargy, weight gain, cold intolerance.
• Neuropsychiatric: poor memory, depression, psychosis, ataxia, slow movements.
• GI: constipation.
• Reproductive: menorrhagia or oligomenorrhoea, infertility.
• Skin/hair: dry skin, thin brittle hair, hair loss, periorbital puffiness.
• Cardio: bradycardia, heart failure, pericardial effusion.
• Neuro exam: “hung-up” slow-relaxing reflexes, proximal myopathy.
• Severe: myxoedema coma (rare, emergency).
• Children: congenital hypothyroidism → growth retardation & cretinism if untreated.

🆚 Differential Diagnosis

• Hypopituitarism (low/normal TSH with low T4).
• Chronic fatigue syndrome.
• Depression (can mimic fatigue/low mood).
• Obstructive sleep apnoea.
• Other causes of weight gain and lethargy (e.g. Cushing’s, anaemia).

🔬 Investigations

• TFTs: high TSH, low T4 (primary); low/normal TSH with low T4 (secondary).
• FBC: normocytic or macrocytic anaemia (pernicious anaemia association).
• CXR: may show pleural/pericardial effusion.
• ECG: bradycardia, low QRS voltage.
• Thyroid antibodies: TPO antibodies positive in autoimmune thyroiditis.
• Ultrasound: if nodules/goitre present.

📊 Classification

• Overt hypothyroidism: TSH ↑ (>10 mU/L), T4 ↓ → treat.
• Subclinical hypothyroidism: TSH ↑, T4 normal. Treat if TSH >10, symptomatic, or TPO-Ab positive.
• Secondary hypothyroidism: T4 ↓ with inappropriately low/normal TSH → pituitary cause.
• Euthyroid: normal TSH & T4.

📉 ECG in Hypothyroidism

🧪 Treatment Algorithm

💊 Management

• Levothyroxine (T4): first-line, lifelong replacement in most patients.
• Dosing: Typical adult dose 100–150 mcg OD. Start lower (25–50 mcg) in elderly or those with ischaemic heart disease, then titrate every 6–8 weeks.
• T3 (liothyronine): only indicated in selected cases (e.g. myxoedema coma, rare poor responders to T4).
• Monitor: TSH (and T4 in secondary hypothyroidism). Target TSH in normal range.
• Pregnancy: higher levothyroxine requirements; check TFTs early each trimester.
• Myxoedema coma: ICU emergency - IV levothyroxine ± liothyronine, IV hydrocortisone, supportive care.

📌 OSCE / Exam Tips

• Classic vignette: young woman with fatigue, cold intolerance, dry skin, TSH ↑, T4 ↓.
• Differentiate subclinical vs overt hypothyroidism.
• In elderly/IHD → start levothyroxine low & slow.
• Myxoedema coma: hypothermia, bradycardia, drowsiness → emergency.
• Don’t forget associations: autoimmune thyroiditis + pernicious anaemia / T1DM.

📚 References

• Hypothyroidism: An Update – AAFP
• Kumar & Clark’s Clinical Medicine, 10th ed.
• NICE CKS: Hypothyroidism (2023).

🧾 Clinical Case Examples – Hypothyroidism

Case 1 – Classic Overt Hypothyroidism 👩‍🦰 A 32-year-old woman presents with 6 months of fatigue, weight gain, constipation, and feeling cold all the time. Exam: dry skin, periorbital puffiness, slow reflexes. Bloods: TSH 18 mU/L, T4 low, TPO antibodies positive. 👉 Diagnosis: Autoimmune hypothyroidism (Hashimoto’s). 👉 Management: Start levothyroxine 100 mcg daily, monitor TSH every 6–8 weeks. Lifelong therapy likely.

Case 2 – Elderly with IHD & Hypothyroidism ❤️ A 75-year-old man with ischaemic heart disease presents with lethargy and ankle swelling. Exam: bradycardia (HR 52), pitting oedema, cool peripheries. Bloods: TSH 12 mU/L, T4 low. 👉 Key point: Elderly patient with cardiac disease - risk of precipitating angina/arrhythmia if started on high-dose thyroxine. 👉 Management: Start low-dose levothyroxine (25 mcg OD), titrate slowly, close cardiac monitoring.

Case 3 – Myxoedema Coma 🚨 A 68-year-old woman with known untreated hypothyroidism is brought in drowsy, hypothermic (34°C), and bradycardic (HR 40). Exam: periorbital puffiness, hypotension, delayed reflexes. Bloods: very low T4, very high TSH, hyponatraemia. 👉 Diagnosis: Myxoedema coma - rare, life-threatening hypothyroidism. 👉 Management: ICU admission, IV levothyroxine ± liothyronine, IV hydrocortisone (until adrenal insufficiency excluded), active warming, fluid/electrolyte correction, treat precipitant (e.g. infection, MI).