Hypothyroidism ✅

Related Subjects: |Thyroid Cancer |Thyroid Eye Disease |Thyroid Function Tests and antibodies |Thyroid Storm - Thyrotoxic crisis |Thyroid Surgery (Thyroidectomy) |Thyroid nodules |Thyrotoxicosis and Hyperthyroidism |Hypothyroidism

💡 Always start with a lower dose of levothyroxine in the elderly or in patients with angina/heart failure to avoid arrhythmia or myocardial ischaemia.

📖 Introduction

• The thyroid secretes mainly T4; most T3 is produced by peripheral conversion.
• Primary hypothyroidism: thyroid failure → TSH ↑, FT4 ↓.
• Secondary/central hypothyroidism: pituitary/hypothalamic disease → TSH low/normal, FT4 ↓; monitor using FT4 and symptoms.

⚙️ Aetiology

• Primary hypothyroidism: autoimmune (Hashimoto's), post-thyroidectomy/radioiodine/radiotherapy, drugs (lithium, amiodarone), thyroiditis, infiltrative, congenital.
• Secondary/central: pituitary/hypothalamic tumours, surgery, radiotherapy, apoplexy, infiltrative disease.

🧑‍⚕️ Clinical Features

• General: fatigue, lethargy, weight gain, cold intolerance ❄️
• Neuropsychiatric: poor concentration, low mood, psychomotor slowing 🧠
• GI: constipation 🍽️
• Reproductive: menorrhagia, subfertility 👩‍🦰
• Skin/hair: dry skin, brittle hair, hair loss, hoarse voice, periorbital puffiness 💇‍♀️
• Cardio: bradycardia, risk of heart failure, pericardial effusion ❤️
• Neuro exam: slow-relaxing reflexes (“hung-up”), proximal myopathy 💪
• Severe: myxoedema coma ❄️🛌 (rare, emergency)

🆚 Differential Diagnosis

• Depression, chronic fatigue, sleep apnoea 😴
• Anaemia, chronic kidney disease, chronic inflammatory disease
• Central hypothyroidism if FT4 low but TSH not elevated

🔬 Investigations (NICE / UK approach)

• First-line: TSH (and FT4 if TSH abnormal or central hypothyroidism suspected)
• Overt primary: TSH ↑, FT4 ↓
• Subclinical: TSH ↑, FT4 normal → repeat in ~3 months
• Thyroid antibodies: TPO Ab supports autoimmune cause
• Other tests: FBC, lipids, B12/folate if indicated
• Imaging: thyroid ultrasound only if structural concerns

📊 Classification

• Overt primary: TSH ↑, FT4 ↓ → treat with levothyroxine 💊
• Subclinical: TSH ↑, FT4 normal → confirm persistence; treat if TSH ≥10 or symptomatic
• Secondary/central: FT4 ↓, TSH low/normal → monitor and treat guided by FT4 & symptoms

🧪 When to Treat Subclinical Hypothyroidism

• TSH ≥10 mIU/L on 2 occasions → levothyroxine
• TSH 4–10 + symptoms → consider 6-month trial; stop if no benefit

💊 Treatment

• First-line: Levothyroxine (T4) monotherapy
• Starting dose: Adults ~50–100 mcg OD; elderly/IHD 25–50 mcg OD → titrate slowly ⬆️
• Take consistently; avoid interaction with iron, calcium, GI disorders
• T3 (liothyronine) rarely used; specialist only
• Central hypothyroidism: titrate using FT4
• Pregnancy: increase dose 25–50%, monitor TSH/FT4 closely 🤰

📈 Monitoring (NICE NG145)

• Primary: TSH every 6–8 weeks until stable → annual
• Persistent symptoms with normal TSH → check FT4
• Central: monitor FT4, TSH unreliable

🚨 Emergencies & Referral

• Myxoedema coma ❄️🛌 → ICU/HDU, IV thyroid hormone + IV hydrocortisone
• Referral: pregnancy, <16y, pituitary disease, difficult TFTs, cardiac disease, drug interactions

📉 ECG Findings

🧪 Treatment Algorithm

🧾 Clinical Cases

Case 1 – Overt primary hypothyroidism 👩‍🦰
32F, fatigue, weight gain, constipation, cold intolerance. TSH 18, FT4 low, TPO Ab +.
👉 Management: start levothyroxine; recheck TSH ~3 monthly until stable, then annual.

Case 2 – Older patient with IHD ❤️
75M, lethargy, bradycardia, TSH 12, FT4 low.
👉 Management: start low-dose levothyroxine 25–50 mcg OD, titrate slowly, monitor for angina/arrhythmia.

Case 3 – Subclinical hypothyroidism 🧪
48M, tiredness, TSH 6.8, FT4 normal.
👉 Next step: repeat TFTs in 3–6 months, check TPO Ab.
👉 Management: trial of levothyroxine if symptoms persist; start if TSH ≥10.

📚 References (UK / NICE)

• NICE NG145: Thyroid disease – Recommendations
• NICE CKS: Hypothyroidism
• NICE CKS: Subclinical hypothyroidism – thresholds, trial, monitoring
• BNF: Levothyroxine sodium – dosing ranges, elderly/cardiac caution
• NHS SPS: Levothyroxine monitoring