9.2 Thyroid Medications
Thyroid pharmacology addresses hypothyroidism with hormone replacement (levothyroxine) and hyperthyroidism with antithyroid drugs (thioamides), iodine, or radioactive iodine.
Thyroid Hormone Replacement
Levothyroxine (T4)
Synthetic T4 (thyroxine). Peripheral conversion to active T3 (triiodothyronine). Preferred over T3 due to longer half-life (7 days) and stable levels.
Indications:
- • Primary hypothyroidism (Hashimoto's, post-ablation)
- • Secondary/tertiary hypothyroidism (pituitary/hypothalamic)
- • TSH suppression in thyroid cancer
- • Myxedema coma (IV T4 or T3)
Monitoring:
- • Target TSH 0.5-5 mIU/L (primary hypothyroidism)
- • Check TSH 6-8 weeks after dose change
- • Take on empty stomach (30-60 min before breakfast)
Adverse (Overdose):
- • Hyperthyroid symptoms (tremor, palpitations, weight loss)
- • Atrial fibrillation, angina
- • Bone loss (chronic excess)
Drug Interactions:
- • ↓ Absorption: Ca²⁺, Fe²⁺, PPIs, bile acid sequestrants
- • ↑ Metabolism: Rifampin, phenytoin, carbamazepine
Antithyroid Drugs (Thioamides)
Propylthiouracil (PTU) & Methimazole
Mechanism of Action:
Inhibit thyroid peroxidase (TPO), blocking oxidation and organification of iodide, preventing coupling of iodotyrosines (MIT/DIT) to form T3/T4. PTU also inhibits peripheral T4→T3 conversion.
Indications:
- • Graves' disease (first-line medical therapy)
- • Toxic multinodular goiter
- • Preparation for radioactive iodine or surgery
- • Thyroid storm (PTU preferred; blocks peripheral conversion)
Methimazole vs PTU:
- • Methimazole: Preferred (longer half-life, once-daily, less hepatotoxic)
- • PTU: Use in 1st trimester pregnancy, thyroid storm
⚠️ Major Toxicities:
- • Agranulocytosis: Rare but serious (0.3%); monitor for fever, sore throat
- • Hepatotoxicity: PTU>methimazole (black box warning for PTU)
- • Skin rash, arthralgia
- • Teratogenic: Methimazole (aplasia cutis), PTU (less severe)
Monitoring: Check CBC if fever/infection symptoms. LFTs at baseline and periodically. TSH/Free T4 every 4-6 weeks.
Iodine Therapy
Radioactive Iodine (I-131)
Emits β particles, destroying thyroid follicular cells. Concentrated in thyroid via NIS (sodium-iodide symporter).
- • Definitive treatment for Graves' disease
- • Toxic nodular goiter
- • Thyroid cancer (ablation of remnant tissue)
- ✓ Most patients become hypothyroid (require levothyroxine)
- ⚠️ Contraindicated in pregnancy/breastfeeding
Iodide (Lugol's Solution, SSKI)
High doses of iodide inhibit thyroid hormone release (Wolff-Chaikoff effect: acute ↓ organification). Effects are temporary (escape phenomenon after 2 weeks).
- • Thyroid storm: Rapid symptom control (with PTU, β-blockers, steroids)
- • Pre-operative preparation for thyroidectomy
- • Radiation protection (nuclear accidents)
- Give PTU 1 hour before iodide (blocks new hormone synthesis first)
Adjunctive Therapies for Hyperthyroidism
Beta-Blockers
- • Propranolol: Preferred (also blocks T4→T3 conversion)
- • Symptomatic relief: Tremor, palpitations, anxiety
- • Does not treat underlying hyperthyroidism
- • Essential in thyroid storm
Corticosteroids
- • Inhibit peripheral T4→T3 conversion
- • Used in thyroid storm
- • Treat thyroid ophthalmopathy (Graves')
Clinical Scenarios
Thyroid Storm (Thyrotoxic Crisis)
Life-threatening emergency: Extreme hyperthyroidism with fever, tachycardia, CNS changes, GI symptoms. Triggered by infection, surgery, trauma in untreated Graves'.
Treatment (Remember: "PTU, Iodide, Propranolol, Prednisolone"):
- 1. PTU (loading dose) — blocks synthesis + peripheral conversion
- 2. Iodide (1 hour after PTU) — blocks release
- 3. Propranolol — symptomatic control + blocks conversion
- 4. Corticosteroids (dexamethasone/hydrocortisone) — blocks conversion, treats adrenal insufficiency
- 5. Supportive care: Cooling, fluids, treat precipitant
Subclinical Hypothyroidism
- • ↑ TSH, normal free T4
- • Treat if: TSH >10 mIU/L, symptoms, pregnancy, cardiovascular disease
- • Start low-dose levothyroxine, titrate to TSH