Prescribing Medicines in Thyroid Disease

PART 1: HYPOTHYROIDISM

First-Line Drug: Levothyroxine (L-T4)

• Young, healthy adults: Start at the full replacement dose - approximately 1.6 mcg/kg/day orally once daily
• Elderly patients or those with cardiac disease: Start LOW at 25-50 mcg/day, then increase by 12.5-25 mcg every 4-6 weeks
• Average maintenance dose: 75-150 mcg/day (varies by body weight, residual thyroid function, and etiology)
• Congenital hypothyroidism (neonates): 10-15 mcg/kg/day - prompt treatment is essential to prevent neurological damage

• Take on an empty stomach, 30-60 minutes before breakfast
• Avoid concurrent calcium, iron supplements, antacids - they reduce absorption

• Check TSH at 6 weeks after initiating or changing dose
• Target TSH: 0.5-2.5 mIU/L for most patients
• Elderly (>65 years): some guidelines accept TSH 1-4 mIU/L
• Once stable, monitor TSH annually

Subclinical Hypothyroidism (elevated TSH, normal T4)

• TSH > 10 mIU/L
• TSH 4.5-10 mIU/L with symptoms, or pregnancy, or age < 65 years with cardiovascular risk factors

When to Add T3 (Liothyronine)?

• Some patients remain symptomatic on T4 monotherapy
• A T4/T3 combination (e.g., adding liothyronine 5-10 mcg) may be considered in selected patients, though evidence is mixed
• Not routinely recommended as first-line

PART 2: HYPERTHYROIDISM

Overall Strategy

Goldman-Cecil Medicine - Treatment of hyperthyroidism

A. Beta-Blockers (Symptom Control - Start First)

• Propranolol (preferred): 20 mg every 6-8 hours, titrate up to maximum 240 mg/day in divided doses. Also inhibits type 1 deiodinase, reducing T3 by ~20%
• Atenolol 25-50 mg/day or Metoprolol 25-50 mg if patient has asthma/COPD (selective agents don't block deiodinase but control symptoms)
• Continue until thyroid hormone levels normalize

B. Antithyroid Drugs (Thionamides)

Methimazole (preferred)

• Initial dose (mild-moderate): 10-20 mg/day as a single oral dose
• Initial dose (severe/storm): 40-60 mg/day in divided doses
• Maintenance: 5-10 mg/day once biochemically controlled
• Duration: 12-18 months for a trial of remission in Graves disease
• Preferred over PTU except in first trimester of pregnancy and thyroid storm

Propylthiouracil (PTU)

• Initial dose: 100-150 mg every 8 hours (severe: up to 200 mg every 4-6 hours)
• Maintenance: 50-100 mg every 8-12 hours
• Advantages: Also blocks peripheral T4-to-T3 conversion - preferred in thyroid storm and first trimester pregnancy
• Disadvantage: Higher risk of hepatotoxicity (rare but serious) vs. methimazole

Monitoring Antithyroid Drugs:

• CBC before starting; recheck if fever or sore throat (agranulocytosis risk ~0.5%)
• Liver function tests - PTU can cause fulminant hepatic failure
• Check TFTs (free T4 + T3) every 4 weeks initially, then every 3-4 months
• Do not use TSH to monitor initially - it remains suppressed for months even after normalization of T4/T3

C. Radioactive Iodine (¹³¹I)

• Not a "prescription drug" per se but an outpatient treatment
• Contraindicated in pregnancy and breastfeeding
• May worsen Graves ophthalmopathy - consider steroid prophylaxis
• Results in hypothyroidism in up to 80% at 1 year - lifelong levothyroxine replacement follows

D. Iodine/Iodide

• Potassium iodide (SSKI) or Lugol's iodine - blocks thyroid hormone release acutely (Wolff-Chaikoff effect)
• Used for pre-operative preparation or in thyroid storm only - not for chronic management

PART 3: THYROID STORM (Thyrotoxic Crisis)

1. PTU 200-300 mg every 4-6 hours (blocks synthesis AND T4-to-T3 conversion)
2. Lugol's iodine 5-10 drops every 8 hours - start 1 hour AFTER PTU (to prevent using iodine as substrate for new hormone)
3. Propranolol 60-80 mg every 4-6 hours orally, or IV esmolol if needed
4. Hydrocortisone 100 mg IV every 8 hours (blocks T4-to-T3 conversion; treats relative adrenal insufficiency)
5. Antipyretics (paracetamol - NOT aspirin, which displaces T4 from binding proteins)
6. Treat the precipitant (infection, surgery, iodine load)

PART 4: SPECIAL POPULATIONS

PART 5: THYROID NODULE / CANCER

• Post-thyroidectomy for cancer: Levothyroxine at TSH-suppressive doses (target TSH 0.1-0.5 mIU/L for low-risk; < 0.1 mIU/L for high-risk differentiated cancer) to inhibit TSH-driven tumor growth
• Benign nodule: No routine suppressive levothyroxine unless there is symptomatic goiter

Summary: At-a-Glance Prescribing

• Goldman-Cecil Medicine (27th ed), Chapter 207 (Pearce EN & Hollenberg AN)