Reference — Endocrine
Thyroid Medications Reference
Levothyroxine, PTU, methimazole, beta-blockers, radioactive iodine, and potassium iodide — mechanisms, administration, monitoring, interactions, and NCLEX focus.
Educational use only. Thyroid medication dosing and protocols vary by provider and institution. Always verify orders and follow facility guidelines. This material supports nursing education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional policy, or medical direction. Always follow facility protocols and current provider orders.
Levothyroxine (Synthroid, Levoxyl)
Thyroid hormone replacement (T4) · Use: Hypothyroidism, myxedema coma (IV formulation), post-thyroidectomy/RAI replacement
| Mechanism | Synthetic T4 — converted to active T3 in peripheral tissues; replaces endogenous thyroid hormone |
| Administration | Oral: 30–60 min before breakfast on empty stomach. IV: for myxedema coma only |
| Monitoring | TSH (primary); free T4; recheck 6–8 weeks after dose changes; annual TSH when stable |
| Interactions | Iron supplements, calcium carbonate, antacids, sucralfate, cholestyramine (separate by 4–6h); warfarin (increases anticoagulant effect); decreased absorption with PPI use |
| Key Notes | NEVER stop abruptly; lifelong in most patients; signs of over-replacement = hyperthyroidism symptoms (tachycardia, palpitations, weight loss); start low and titrate in elderly/cardiac patients; requirements increase in pregnancy |
| NCLEX Focus | Take on EMPTY STOMACH 30–60 min before breakfast. Separate iron/calcium by at least 4 hours. TSH is the monitoring test. Signs of too much = tachycardia, anxiety — report to provider. |
Propylthiouracil (PTU)
Antithyroid drug — thionamide · Use: Hyperthyroidism (Graves' disease), thyroid storm (PREFERRED over methimazole), 1st trimester pregnancy
| Mechanism | Blocks thyroid hormone synthesis by inhibiting thyroid peroxidase; ALSO inhibits peripheral conversion of T4 → T3 (unique advantage in thyroid storm) |
| Administration | Oral, given 3 times daily (shorter half-life than methimazole). For thyroid storm: administered via NG tube if patient cannot swallow. |
| Monitoring | CBC with differential (agranulocytosis risk); LFTs (hepatotoxicity — more common with PTU than methimazole); thyroid function tests (TSH, free T4) |
| Interactions | Warfarin interaction (monitor INR); additive with other hepatotoxic drugs |
| Key Notes | Black Box Warning: hepatotoxicity (hepatic failure reported — serious risk). Preferred in thyroid storm (dual mechanism). Preferred in 1st trimester (methimazole teratogenic in early pregnancy). Switch to methimazole after 1st trimester. |
| NCLEX Focus | PTU preferred in THYROID STORM (blocks both synthesis AND T4→T3 conversion). Report fever/sore throat/mouth sores immediately — agranulocytosis. Black Box: hepatotoxicity. |
Methimazole (Tapazole)
Antithyroid drug — thionamide · Use: Hyperthyroidism (preferred for most non-pregnant patients with Graves' disease or toxic goiter)
| Mechanism | Blocks thyroid peroxidase → inhibits thyroid hormone synthesis; does NOT inhibit T4→T3 conversion (unlike PTU) |
| Administration | Oral, once or twice daily (longer half-life than PTU — better adherence); used long-term before RAI or surgery |
| Monitoring | CBC (agranulocytosis), LFTs (hepatotoxicity — less common than PTU), thyroid function (TSH, free T4) |
| Interactions | Warfarin interaction; additive agranulocytosis risk with other agents |
| Key Notes | Preferred over PTU for most hyperthyroid patients (once-daily dosing, lower hepatotoxicity risk). CONTRAINDICATED in 1st trimester pregnancy (methimazole embryopathy — choanal atresia, aplasia cutis). Safe after 1st trimester. |
| NCLEX Focus | Methimazole preferred for most hyperthyroid patients EXCEPT thyroid storm (no T4→T3 conversion block) and 1st trimester. Agranulocytosis: report fever/sore throat immediately. |
Propranolol (Inderal) / Atenolol (Tenormin)
Beta-adrenergic blockers · Use: Symptom control in hyperthyroidism (tachycardia, palpitations, tremor, anxiety); thyroid storm (first drug given)
| Mechanism | Block beta-adrenergic stimulation of heart, reducing tachycardia and palpitations. Propranolol also inhibits T4→T3 conversion at high doses. |
| Administration | Oral for chronic use; propranolol IV for thyroid storm |
| Monitoring | Heart rate, blood pressure, respiratory status |
| Interactions | Standard beta-blocker interactions; caution in asthma, COPD, bradycardia, heart block; mask hypoglycemia symptoms |
| Key Notes | Beta-blockers do NOT treat hyperthyroidism — they only control symptoms. Patient still needs antithyroid drugs or RAI for definitive treatment. Propranolol is preferred in thyroid storm due to T4→T3 conversion inhibition. |
| NCLEX Focus | First drug given in thyroid storm. Controls symptoms ONLY — does NOT reduce thyroid hormone levels. Propranolol preferred over atenolol in thyroid storm. |
Radioactive Iodine (RAI, I-131)
Thyroid ablation therapy · Use: Definitive treatment for Graves' disease and toxic nodular goiter; thyroid cancer ablation post-thyroidectomy
| Mechanism | I-131 is taken up by thyroid follicular cells, emits beta radiation → destroys thyroid tissue over 6–12 weeks |
| Administration | Oral (capsule or liquid); outpatient in most cases; patient given radiation safety instructions |
| Monitoring | Thyroid function every 4–8 weeks post-RAI; most patients develop hypothyroidism requiring lifelong levothyroxine |
| Interactions | Antithyroid drugs should be stopped 3–5 days before RAI (can reduce uptake); avoid iodine-rich foods and contrast for 2 weeks before |
| Key Notes | Contraindicated in pregnancy, breastfeeding, active thyroid eye disease (may worsen). Post-RAI radiation precautions: avoid close contact with pregnant women/children for 1 week; avoid sharing utensils, flush toilet twice, sleep alone for several days per instructions. |
| NCLEX Focus | Most patients become HYPOTHYROID after RAI — need lifelong levothyroxine. Contraindicated in PREGNANCY. Radiation precautions post-treatment (isolation from pregnant women and children). |
Potassium Iodide (Lugol's Solution, SSKI)
Thyroid hormone synthesis and release inhibitor · Use: Thyroid storm (given 1 hour AFTER PTU or methimazole); preoperative thyroid preparation; radiation emergency protection
| Mechanism | Wolff-Chaikoff effect: high iodine dose temporarily inhibits iodine organification and thyroid hormone RELEASE from gland |
| Administration | Oral liquid; must be given at least 1 hour after antithyroid drug (PTU/methimazole) to prevent providing substrate for new hormone synthesis |
| Monitoring | Thyroid function, signs of iodism (metallic taste, salivation, rash, GI upset — rare with short-term use) |
| Interactions | Additive with other antithyroid agents |
| Key Notes | CRITICAL: Give antithyroid drug (PTU) FIRST, then potassium iodide 1 hour later. If iodide given without antithyroid drug first, it provides iodine substrate for thyroid hormone synthesis and can worsen thyrotoxicosis. |
| NCLEX Focus | PTU FIRST → potassium iodide 1h LATER (not simultaneously, not reversed). Iodide given alone worsens hyperthyroidism by providing substrate. |
Related Resources
Standards & sources
Fact-checked Jun 20, 2026This page is written to align with American Diabetes Association (ADA) Standards of Care · American Association of Clinical Endocrinology (AACE). It is an educational summary, not a citation of any single document — always verify specific doses, values, and protocols against current guidelines and your facility policy. How we source content →