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 and in those with angina or heart failure, to avoid precipitating arrhythmia or ischaemia.

🦋 Hypothyroidism is underactivity of the thyroid gland leading to inadequate thyroid hormone effect at tissue level. In the UK, the commonest cause is autoimmune thyroiditis (Hashimoto’s), while iodine deficiency remains the most common cause worldwide. Clinically, always think in two frames: primary (thyroid failure: TSH usually rises) versus secondary/central (pituitary/hypothalamic disease: TSH may be low/normal and is not a reliable marker of replacement adequacy).

📖 Introduction

• The thyroid secretes mainly T4 and smaller amounts of T3; most T3 is produced by peripheral conversion of T4.
• Primary hypothyroidism: raised TSH is a sensitive marker; FT4 distinguishes overt vs subclinical disease.
• Secondary (central) hypothyroidism: suspect when FT4 is low with TSH low/normal; monitor and dose-adjust using FT4 (not TSH) and the clinical picture.

⚙️ Aetiology

• Primary hypothyroidism (most cases): autoimmune thyroiditis, post-thyroidectomy, post-radioiodine, external neck radiotherapy, drugs, iodine deficiency.
• Secondary/central hypothyroidism: pituitary or hypothalamic disease (tumours, surgery/radiotherapy, infiltrative disease, apoplexy, etc.).

🧬 Causes of Primary Hypothyroidism

• Autoimmune: Hashimoto’s (± goitre) or atrophic thyroiditis; often associated with other autoimmune conditions.
• Post-ablative: thyroidectomy, radioiodine, external radiotherapy.
• Drugs: lithium, amiodarone, antithyroid drugs, interferons (and others).
• Thyroiditis: subacute (de Quervain), painless/post-viral, postpartum thyroiditis (often transient course).
• Infiltrative: haemochromatosis, sarcoid, amyloid (rare).
• Congenital: dysgenesis/ectopic gland or dyshormonogenesis (newborn screening is key).

🧑‍⚕️ Clinical Features

• General: fatigue, lethargy, weight gain, cold intolerance.
• Neuropsychiatric: poor concentration/memory, low mood, psychomotor slowing.
• GI: constipation.
• Reproductive: menstrual disturbance (often menorrhagia), subfertility.
• Skin/hair: dry skin, brittle hair, hair loss, hoarse voice; periorbital puffiness.
• Cardio: bradycardia; can contribute to heart failure; pericardial effusion (classically severe/longstanding).
• Neuro exam: slow-relaxing (“hung-up”) reflexes; proximal myopathy.
• Severe: myxoedema coma (rare, life-threatening emergency).

🆚 Differential Diagnosis

• Depression, sleep apnoea, chronic fatigue syndromes.
• Anaemia, chronic kidney disease, chronic inflammatory disease.
• Central hypothyroidism (consider if FT4 low with non-elevated TSH, or other pituitary features).

🔬 Investigations (NICE / UK approach)

• First-line: TSH (and FT4 if TSH abnormal or central disease suspected).
• Primary overt hypothyroidism: TSH high with FT4 low.
• Subclinical hypothyroidism: TSH high with FT4 normal; in non-pregnant adults, repeat TFTs in 3–6 months to confirm persistent abnormality and exclude transient causes.
• Thyroid antibodies: TPO antibodies support autoimmune aetiology and predict progression in subclinical disease.
• Associated tests (case-dependent): FBC (anaemia), lipids (secondary dyslipidaemia), B12/folate if macrocytosis or autoimmune clustering suspected.
• Imaging: thyroid ultrasound only if goitre/nodules or structural concern (not for routine “Hashimoto’s confirmation”).

📊 Classification (practical, exam-safe)

• Overt primary hypothyroidism: TSH ↑ with FT4 ↓ → treat with levothyroxine.
• Subclinical hypothyroidism (non-pregnant): TSH ↑ with FT4 normal → confirm persistence before long-term decisions.
• Secondary/central hypothyroidism: FT4 ↓ with inappropriately low/normal TSH → assess pituitary and treat/monitor by FT4.

🧪 When to treat subclinical hypothyroidism (NICE-aligned)

• Consider levothyroxine if TSH ≥10 mIU/L on two separate occasions (typically 3 months apart).
• Consider a 6-month trial of levothyroxine if TSH above reference but <10 on two occasions and the person has symptoms suggestive of hypothyroidism (review benefit and stop if no symptomatic improvement and no other indication).

📉 ECG in Hypothyroidism

🧪 Treatment Algorithm

💊 Management

• First-line: Levothyroxine (T4) monotherapy for primary hypothyroidism.
• Starting dose (adult): depends on age, comorbidity and severity. Many adults start around 50–100 micrograms once daily, then adjust.
• Older adults and/or ischaemic heart disease: start low and go slow (e.g. 25–50 micrograms daily) and titrate cautiously.
• How to take: take consistently each day; absorption can be reduced by iron, calcium, bile acid sequestrants and some GI conditions—separate dosing if needed.
• T3 (liothyronine): not routine; specialist use only in selected situations.

📈 Monitoring while on levothyroxine (NICE NG145)

• Primary hypothyroidism: consider checking TSH every 3 months until stable (defined as 2 similar TSH results within range, 3 months apart), then once yearly.
• If symptoms persist despite a normal TSH, consider measuring FT4 as well as TSH.
• Central hypothyroidism: monitor and titrate using FT4 and symptoms (TSH is unreliable as a target).

🚨 Emergencies & referral (UK practice)

• Myxoedema coma (hypothermia, bradycardia, hypotension, altered mental state, hyponatraemia) → blue-light emergency, ICU/HDU, IV thyroid hormone per specialist protocol, and IV hydrocortisone until adrenal insufficiency excluded.
• Refer/seek specialist input if: pregnant or planning pregnancy, age <16, suspected pituitary disease, difficult-to-control TFTs, significant cardiac disease, or drug interactions (e.g. amiodarone/lithium).

🧾 Clinical case examples

Case 1 – Overt primary hypothyroidism 👩‍🦰
32-year-old with fatigue, weight gain, constipation, cold intolerance. Exam: dry skin, periorbital puffiness, slow reflexes. Bloods: TSH 18 mIU/L, FT4 low, TPO Ab positive.
👉 Diagnosis: autoimmune primary hypothyroidism.
👉 Management: start levothyroxine; recheck TSH ~3 monthly until stable, then annual.

Case 2 – Older patient with IHD ❤️
75-year-old with IHD and lethargy, bradycardia. Bloods: TSH 12 mIU/L, FT4 low.
👉 Key point: risk of angina/arrhythmia if up-titrated too fast.
👉 Management: start low-dose levothyroxine (e.g. 25–50 micrograms OD) and titrate slowly with monitoring.

Case 3 – Subclinical hypothyroidism 🧪
48-year-old, tiredness. Bloods: TSH 6.8 mIU/L, FT4 normal.
👉 Next step: repeat TFTs in 3–6 months to confirm persistence and consider TPO antibodies.
👉 Treatment decision: if TSH remains raised and symptoms persist, consider a time-limited trial; if TSH ≥10 on repeat, consider treatment.

📚 References (UK / NICE)

• NICE NG145: Thyroid disease – Recommendations (monitoring on levothyroxine; when to measure TSH/FT4)
• NICE CKS: Hypothyroidism (assessment, diagnosis, management)
• NICE CKS: Subclinical hypothyroidism (non-pregnant) – thresholds, trial of treatment, monitoring
• BNF (NICE): Levothyroxine sodium (dosing ranges; elderly/cardiac cautious initiation)
• NHS Specialist Pharmacy Service: Levothyroxine monitoring (practical monitoring/titration summary)