Thyroid drugs compared — hypothyroid vs hyperthyroid, methimazole vs PTU, and thyroid storm
Verified · Oct 2026Thyroid hormone is made, stored, released and converted from T4 to active T3. Replacement adds hormone back; every antithyroid drug works at one of those steps, and that step decides how fast it acts.
One pathway, a drug at each step
Thyroid peroxidase (TPO) iodinates thyroglobulin and couples MIT and DIT into T3 and T4, which are stored on thyroglobulin and released by proteolysis. Thyroid receptors bind T3 with much higher affinity than T4, and about 80% of T3 comes from deiodination of T4 in the tissues.
For hypothyroidism, levothyroxine is synthetic T4, converted to T3 by deiodinases; its long half-life (6 to 7 days) means TSH is rechecked 6 to 8 weeks after a dose change. Liothyronine is T3 itself: it acts within hours, with a maximal response in 2 to 3 days.
For hyperthyroidism, the thionamides (methimazole, propylthiouracil) inhibit TPO but do not inactivate hormone already stored or circulating, so reaching euthyroidism typically takes several months. Pharmacologic iodide is the fastest — hours to days — because it blocks hormone release; it also transiently inhibits organification (the Wolff–Chaikoff effect), but the gland escapes within 2 to 4 weeks. Radioactive iodine (I-131) is taken up through the sodium-iodide symporter and destroys thyroid tissue by beta emission; the effect takes months, and the usual long-term result is hypothyroidism. Beta-blockers control the adrenergic symptoms, and propranolol at higher doses also inhibits T4-to-T3 conversion.
Thyroid storm: the order matters
Thyroid storm is an acute, life-threatening complication of hyperthyroidism, often precipitated by stopping antithyroid medicine, thyroid surgery, infection, iodinated contrast or radioiodine. Alongside supportive care — IV fluids, oxygen, cooling, acetaminophen — each drug targets one step.
A beta-blocker comes first, for the adrenergic surge. A thionamide then stops new synthesis; propylthiouracil is favored because it also blocks peripheral T4-to-T3 conversion, with a switch to methimazole after stabilization. Iodine follows, at least an hour after the thionamide — given first, the iodine load would fuel new hormone synthesis. Conversion blockers include a glucocorticoid (hydrocortisone), PTU, propranolol and iodinated contrast, and a bile acid sequestrant can interrupt enterohepatic recycling in severe cases. Aspirin is avoided, because it can raise free thyroid hormone.
Two boxed warnings frame the class: levothyroxine and liothyronine are not for obesity or weight loss, and propylthiouracil carries a box for severe liver injury and acute liver failure.
Comparison
Drugs for an underactive or overactive thyroid
| Drug | Mechanism | Speed of effect | Signature risk |
|---|---|---|---|
| Levothyroxine | Synthetic T4, converted to active T3 by deiodinases | Slow: half-life 6–7 days; TSH 6–8 weeks after a change | Over-replacement → hyperthyroid effects (angina, arrhythmia, bone loss); boxed: not for weight loss |
| Liothyronine | T3 itself | Within hours; maximal at 2–3 days | Hyperthyroid effects; boxed: not for weight loss |
| Methimazole | Inhibits TPO (iodination and coupling) | Several months to euthyroidism (stored hormone untouched) | Agranulocytosis; first-trimester birth defects |
| Propylthiouracil | Inhibits TPO; also blocks peripheral T4 → T3 | Several months to euthyroidism | Boxed: severe liver injury; agranulocytosis |
| Iodide (SSKI, Lugol’s) | Blocks hormone release; transient Wolff–Chaikoff block | Fastest — hours to days; escape in 2–4 weeks | Jod-Basedow hyperthyroidism from an iodine load |
| Sodium iodide I-131 | Taken up by NIS; beta emission destroys thyroid tissue | Months | Radiation thyroiditis; usual long-term hypothyroidism; contraindicated in pregnancy |
| Beta-blockers | Block adrenergic effects; propranolol also ↓ T4 → T3 | — | — |
— = not stated on the source pages.
Methimazole vs propylthiouracil
| Drug | Extra action | When preferred | Liver | Pregnancy | Children | Kinetics |
|---|---|---|---|---|---|---|
| Methimazole | — | First-line: equally effective, once daily, better safety | Cholestatic injury | Avoided in the first trimester (aplasia cutis, choanal or esophageal atresia); used in the second and third | Used; pediatric liver failure not reported | < 10% protein-bound; half-life 4–6 h; crosses the placenta readily |
| Propylthiouracil | Blocks peripheral T4 → T3 | Thyroid storm; just before or in the first trimester; methimazole intolerance when RAI or surgery is unsuitable | Boxed severe liver injury and acute liver failure (allergic hepatitis) | Treatment of choice just before and during the first trimester | Not recommended (liver failure, transplant, death) | 80–85% bound; half-life about 1 h |
Both can cause agranulocytosis (idiosyncratic, mostly in the first 3 months; they cross-react) and ANCA-positive vasculitis.
— = not stated on the source pages.
Thyroid storm, step by step
| Step | Drug | What it blocks | Note |
|---|---|---|---|
| 1 · Adrenergic control | Beta-blocker (propranolol) | Adrenergic effects; propranolol also ↓ T4 → T3 | Started first |
| 2 · Stop synthesis | Thionamide — PTU favored | New hormone synthesis (PTU also conversion) | Switch to methimazole after stabilization |
| 3 · Stop release | Iodine solution (SSKI) | Release of stored hormone | Only after the thionamide — the iodine load would otherwise fuel synthesis |
| 4 · Block conversion | Glucocorticoid (hydrocortisone) | Peripheral T4 → T3 | Iodinated contrast, PTU and propranolol also block conversion |
| 5 · Interrupt recycling | Bile acid sequestrant (cholestyramine) | Enterohepatic recycling | Severe cases |
Avoid aspirin — it can raise free thyroid hormone.
High-yield
Practice questions
Q1A 38-year-old woman with Graves disease, already taking methimazole, is scheduled for thyroidectomy. Her surgeon wants the drug that acts fastest on her thyroid, to reduce gland vascularity and blood loss.
Which drug fits?
- A.A higher dose of methimazole
- B.Radioactive iodine
- C.A saturated solution of potassium iodide (SSKI) or Lugol’s solution
- D.Levothyroxine
Show answer
C. A saturated solution of potassium iodide (SSKI) or Lugol’s solution — Pharmacologic iodide is the fastest-acting antithyroid drug, working in hours to days by blocking hormone release, and SSKI or Lugol’s solution is given before surgery in Graves disease to reduce gland vascularity and blood loss. Thionamides do not touch stored hormone and take months to restore euthyroidism; radioactive iodine also takes months and destroys the gland rather than preparing it; levothyroxine is T4 replacement, which would add hormone.
Q2A 45-year-old man with newly diagnosed Graves disease, not in thyroid storm, is starting an antithyroid drug.
Which statement best supports methimazole over propylthiouracil?
- A.Methimazole also blocks peripheral T4-to-T3 conversion
- B.Methimazole is equally effective, given once daily, with a better safety profile
- C.Methimazole carries no risk of agranulocytosis
- D.Methimazole is preferred in the first trimester of pregnancy
Show answer
B. Methimazole is equally effective, given once daily, with a better safety profile — Methimazole is preferred first-line because it is equally effective, given once daily and safer; propylthiouracil carries a boxed warning for severe liver injury and is reserved for specific situations. Peripheral T4-to-T3 blockade is propylthiouracil’s extra action; agranulocytosis occurs with both, and they cross-react; in the first trimester propylthiouracil may be the treatment of choice, because methimazole is linked to birth defects.
Q3A 34-year-old woman started methimazole 6 weeks ago. She now has a fever of 39 °C and a sore throat.
What is the most appropriate action?
- A.Continue methimazole and treat a presumed viral pharyngitis
- B.Switch to propylthiouracil and continue treatment
- C.Reassure her, since routine monthly blood counts would already have shown a problem
- D.Stop methimazole and check a white blood cell count
Show answer
D. Stop methimazole and check a white blood cell count — Agranulocytosis is an idiosyncratic thionamide reaction, usually in the first 3 months; fever or sore throat means stopping the drug and checking a white count. PTU and methimazole cross-react, so switching does not remove the risk, and routine granulocyte monitoring is considered unnecessary — the reaction is caught by symptoms, not scheduled counts.
Q4A 52-year-old woman with hypothyroidism is switched from levothyroxine to liothyronine. Her symptoms change within hours of the first doses, much faster than they did on levothyroxine.
What best explains the faster response?
- A.Liothyronine is converted to T4 in the tissues
- B.Liothyronine has a longer half-life than levothyroxine
- C.Liothyronine is T3 itself, the active hormone
- D.Liothyronine inhibits thyroid peroxidase
Show answer
C. Liothyronine is T3 itself, the active hormone — Liothyronine is T3, which acts within hours with a maximal response in 2 to 3 days; levothyroxine is T4, converted to active T3 by deiodinases, and it has a long half-life (6 to 7 days, against 2 days or less for T3). Conversion runs from T4 to T3, not the reverse, and inhibiting thyroid peroxidase is how the thionamides act.
Sources
- Physiology, Thyroid Hormone — StatPearls (NCBI)
- Methimazole — StatPearls (NCBI)
- Propylthiouracil — StatPearls (NCBI)
- Potassium Iodide — StatPearls (NCBI)
- Radioactive Iodine (I-131) Therapy for Hyperthyroidism and Thyroid Cancer — StatPearls (NCBI)
- Thyroid Storm — StatPearls (NCBI)
- Hyperthyroidism — StatPearls (NCBI)
- Myxedema Coma — StatPearls (NCBI)
- Levothyroxine — StatPearls (NCBI)
- Synthroid (levothyroxine) — FDA label — FDA / DailyMed
- Propylthiouracil tablets — FDA label — FDA / DailyMed
- Methimazole tablets — FDA label — FDA / DailyMed
- Cytomel (liothyronine) — FDA label — FDA / DailyMed
- HICON (sodium iodide I-131) — FDA label — FDA / DailyMed
- IOSAT (potassium iodide) — FDA label — FDA / DailyMed
- ThyroSafe (potassium iodide) — FDA label — FDA / DailyMed
- Propranolol — StatPearls (NCBI)
- Inderal LA (propranolol ER) — FDA label — FDA / DailyMed
Each class page compared lists its full sources; this page reuses facts verified there.
By LearnDrugs
Educational summary for medical students. Always verify against current prescribing information and guidelines. Not medical advice.