Propylthiouracil
propylthiouracil (PTU)
Indications
| Indication | Approved Population | Therapy Type | Status |
|---|---|---|---|
| Graves’ disease / hyperthyroidism — when methimazole is not tolerated and surgery or radioactive iodine is not appropriate | Adults | Monotherapy | FDA Approved |
| Toxic multinodular goiter — when methimazole is not tolerated and surgery or RAI is not appropriate | Adults | Monotherapy | FDA Approved |
| Pre-thyroidectomy / pre-RAI preparation — symptom control before definitive therapy, in patients intolerant of methimazole | Adults | Short-term bridging | FDA Approved |
| Hyperthyroidism in first trimester pregnancy — preferred antithyroid drug during organogenesis due to lower teratogenic risk vs methimazole | Pregnant adults (1st trimester) | Monotherapy | FDA Approved |
Propylthiouracil occupies a second-line position among antithyroid drugs. The ATA 2016 guidelines recommend methimazole as the first-choice thioamide in virtually all non-pregnant adults; propylthiouracil is reserved for patients who cannot tolerate methimazole, those in the first trimester of pregnancy, and those experiencing thyroid storm. The FDA added a boxed warning in 2010 restricting propylthiouracil use due to reports of severe hepatotoxicity, and the drug is no longer recommended for routine pediatric use.
Thyroid storm / thyrotoxic crisis: The ATA recommends propylthiouracil as the preferred thioamide in thyroid storm because it blocks peripheral T4-to-T3 conversion in addition to inhibiting thyroid hormone synthesis. Loading dose of 500–1000 mg followed by 250 mg every 4 hours is widely cited. Evidence quality: Moderate (guideline-supported, no large RCTs).
Graves’ disease — initial treatment (when methimazole tolerated): Although methimazole is preferred, propylthiouracil 50–150 mg three times daily has been used as initial therapy in Graves’ disease at some centres. Evidence quality: High (RCT data available; ATA recommends methimazole first).
Dosing
Adult Dosing by Clinical Scenario
| Clinical Scenario | Starting Dose | Maintenance Dose | Maximum Dose | Notes |
|---|---|---|---|---|
| Mild–moderate hyperthyroidism (Graves’ or TMNG) — methimazole-intolerant | 100 mg TID | 50–150 mg/day | 600 mg/day | Divide total daily dose q8h; titrate to normalise TSH and FT4 Aim for lowest effective dose once euthyroid |
| Severe hyperthyroidism with large goiter | 150–200 mg TID | 100–150 mg/day | 900 mg/day | Higher initial doses occasionally required; reduce once FT4 trending down FDA PI: 600–900 mg/day may be needed initially |
| Pre-thyroidectomy or pre-RAI bridging | 100 mg TID | Titrate to euthyroid | 600 mg/day | Discontinue 3–5 days before RAI; continue until surgery for thyroidectomy Stop PTU in the week before/after RAI to avoid reduced RAI efficacy (ATA 2016) |
| Thyroid storm (off-label) | 500–1000 mg load | 250 mg q4h | 1500 mg/day | Rectal or NG administration if oral route unavailable Administer ≥1 h before iodine to prevent iodine organification (ATA 2016) |
| First-trimester pregnancy with Graves’ disease | 50–100 mg TID | Lowest effective dose | 300 mg/day | Target FT4 at or just above upper limit of normal; switch to methimazole after week 16 Thyroid dysfunction often improves as pregnancy advances — consider dose reduction or cessation |
Propylthiouracil has a short plasma half-life (~1–2 hours), but its intrathyroidal duration of action extends to 12–24 hours because the drug concentrates in thyroid tissue. Despite this, TID dosing (every 8 hours) remains standard for initial therapy to ensure consistent suppression of hormone synthesis. Once euthyroid, some patients can be managed on twice-daily dosing, though evidence for this approach is limited.
Pharmacology
Mechanism of Action
Propylthiouracil belongs to the thioamide class of antithyroid drugs and exerts its therapeutic effect through two complementary mechanisms. First, it inhibits thyroid peroxidase (TPO), the enzyme responsible for oxidising iodide to iodine and coupling iodine to tyrosine residues on thyroglobulin. By blocking this organification step, propylthiouracil prevents the formation of monoiodotyrosine (MIT) and diiodotyrosine (DIT) and, consequently, the synthesis of thyroxine (T4) and triiodothyronine (T3). Second, and unlike methimazole, propylthiouracil inhibits the peripheral deiodinase enzyme (type 1 5′-deiodinase) that converts T4 to the more biologically potent T3. This dual mechanism makes propylthiouracil particularly useful in thyroid storm, where rapid reduction of circulating T3 is critical. The drug does not affect thyroid hormone that has already been synthesised and stored, so clinical effect is delayed 24–36 hours after the first dose, and euthyroidism typically takes 2–4 months to achieve.
ADME Profile
| Parameter | Value | Clinical Implication |
|---|---|---|
| Absorption | Rapid oral absorption; Tmax 1–2 h; bioavailability 53–88% | Variable bioavailability may contribute to unpredictable dose-response; food effect not well characterised |
| Distribution | Vd ~0.4 L/kg (~30 L); 75–85% protein-bound (albumin, lipoproteins); concentrates in thyroid gland | High protein binding limits transfer to breast milk; intrathyroidal concentration prolongs pharmacological action beyond plasma half-life |
| Metabolism | Extensive hepatic metabolism via glucuronidation and sulfation; <10% excreted unchanged | Hepatic metabolism raises concern for hepatotoxicity; no identified CYP-mediated interactions; clearance ~120 mL/min/m² |
| Elimination | t½ = 1–2 h; ~35% recovered in urine within 24 h as metabolites; some biliary excretion with enterohepatic recycling | Short half-life necessitates TID dosing; half-life not significantly altered by thyroid status or age; no dose adjustment for renal impairment |
Side Effects
Propylthiouracil adverse reaction data are derived primarily from postmarketing reports rather than placebo-controlled trials, making precise incidence figures less definitive than for newer agents. The frequencies below draw on the FDA PI, observational cohort studies, and published case series. The overall adverse event rate with propylthiouracil has been reported at approximately 52% in one RCT comparing it to methimazole.
| Adverse Effect | Incidence | Clinical Note |
|---|---|---|
| Transient AST/ALT elevation | ~27% | Usually subclinical and transient; majority resolve without dose change (Nakamura et al.). Distinguish from fulminant hepatotoxicity. |
| Skin rash / urticaria | ~15% | Typically mild pruritic maculopapular rash; may resolve with antihistamines without stopping therapy. Cross-reactivity with methimazole possible (~50%). |
| Adverse Effect | Incidence | Clinical Note |
|---|---|---|
| Arthralgia / myalgia | ~5–8% | Joint and muscle pain reported frequently; distinguish from autoimmune flare |
| Gastrointestinal disturbance (nausea, epigastric distress) | ~5% | Taking with food may alleviate symptoms; generally dose-independent |
| Abnormal hair loss (alopecia) | ~4% | May also occur from hyperthyroidism itself; usually reversible upon discontinuation |
| Headache / drowsiness | ~3–5% | Usually mild and self-limiting |
| Loss of taste / taste perversion | ~2–3% | Typically reversible after discontinuation |
| Leukopenia (mild, <4000/mm³) | ~1.8% | Distinguish from transient leukopenia of untreated hyperthyroidism (~10% of Graves’ patients); check baseline WBC before starting |
| Paresthesias / neuritis | ~1–2% | Peripheral sensory symptoms; evaluate for B12 deficiency or other causes |
| Adverse Effect | Estimated Frequency | Typical Onset | Required Action |
|---|---|---|---|
| Severe hepatotoxicity / acute liver failure | ~1 in 10,000 | Any time (often first 6 months); onset is rapid and unpredictable | Immediate discontinuation; LFTs; hepatology referral; liver transplant may be required. FDA identified 34 cases (13 deaths, 5 transplants) through 2009. |
| Agranulocytosis (ANC <500/mm³) | 0.3–0.8% | Usually within first 90 days; median 28 days (range 14–2989 days) | Stop PTU immediately; obtain CBC with differential; supportive care with broad-spectrum antibiotics; consider G-CSF. Never rechallenge with any thioamide. |
| ANCA-positive vasculitis (MPO-ANCA) | ANCA positivity up to 27%; clinical vasculitis rare | Months to years (mean ~42 months of therapy) | Discontinue PTU; ANCA titres, urinalysis, renal function; may require corticosteroids or cyclophosphamide for severe organ involvement |
| Aplastic anaemia / pancytopenia | Very rare | Any time during therapy | Stop PTU; haematology referral; bone marrow biopsy; supportive transfusion |
| Lupus-like syndrome | Rare | Weeks to months | Discontinue PTU; ANA screen; symptoms usually resolve after withdrawal |
| Stevens-Johnson syndrome / toxic epidermal necrolysis | Very rare | Days to weeks after initiation | Immediate discontinuation; dermatology/burns referral; supportive care |
| Interstitial pneumonitis | Very rare | Variable | Stop PTU; chest imaging; consider corticosteroids if confirmed |
| Reason for Discontinuation | Incidence | Context |
|---|---|---|
| Hepatotoxicity (clinical or biochemical) | ~3–5% | Most common serious reason; transaminase elevation >2× ULN was significantly more frequent with PTU than methimazole |
| Skin reactions | ~3–4% | When rash does not resolve with antihistamines and dose reduction |
| Agranulocytosis / leukopenia | ~0.5–1% | Mandates permanent discontinuation; higher at PTU 300 mg/day (~0.81%) than lower doses |
| Arthralgia / myalgia | ~1–2% | When musculoskeletal symptoms are persistent and limit daily activities |
Unlike most drug-induced liver injuries, propylthiouracil hepatotoxicity is idiosyncratic with rapid, unpredictable onset. Routine biochemical monitoring of liver function does not reliably prevent fulminant hepatic failure. Instead, patient education is the primary safety measure: instruct patients to report anorexia, pruritus, right upper quadrant pain, dark urine, pale stools, or jaundice immediately, particularly during the first six months. If these symptoms occur, discontinue propylthiouracil without waiting for laboratory results.
Drug Interactions
Propylthiouracil does not undergo significant CYP450-mediated metabolism and has few direct pharmacokinetic interactions. However, it has a clinically important pharmacodynamic interaction with oral anticoagulants and several indirect interactions mediated by its restoration of euthyroid state, which alters the clearance of many drugs.
Monitoring
-
Thyroid Function (TSH, FT4)
Every 4–6 weeks until euthyroid; then every 3–6 months
Routine Aim to maintain TSH within normal range and FT4 at or just above upper limit of normal (especially in pregnancy). Elevated TSH during therapy signals over-treatment — reduce dose. FT3 may be useful in thyroid storm. -
CBC with Differential
Baseline; then if symptoms arise
Trigger-based Obtain baseline WBC and differential before starting therapy; repeat immediately if patient develops sore throat, fever, or mouth ulcers. Routine periodic CBC has not been shown to prevent agranulocytosis due to its sudden onset, but the ATA recommends baseline measurement. -
Liver Function (ALT, AST, bilirubin)
Baseline; then if symptoms arise
Trigger-based Routine LFT monitoring does not prevent fulminant hepatic failure due to its unpredictable onset (FDA PI). Obtain baseline; thereafter, test immediately if patient reports anorexia, right upper quadrant pain, dark urine, or jaundice. Discontinue PTU if ALT/AST >3× ULN with symptoms. -
PT / INR
If on concurrent anticoagulants; before surgery
Trigger-based PTU has intrinsic anti-vitamin K activity that can potentiate warfarin. Monitor INR frequently during PTU initiation and any dose change in patients on oral anticoagulants. -
TRAb (TSH Receptor Antibody)
After 12–18 months of therapy
Routine In Graves’ disease, TRAb level guides decision to stop antithyroid therapy. Persistently elevated TRAb predicts relapse after discontinuation. ATA 2016 recommends measuring TRAb before planned cessation of ATD therapy. -
Pregnancy Testing
Before starting; if pregnancy suspected
Trigger-based PTU is preferred only in the first trimester. Women of childbearing potential should be counselled about the need to contact their prescriber immediately if pregnancy occurs, as a switch to methimazole after week 16 is recommended.
Contraindications & Cautions
Absolute Contraindications
- Known hypersensitivity to propylthiouracil or any component of the formulation
- Prior PTU-induced agranulocytosis or severe hepatotoxicity — never rechallenge
Relative Contraindications (Specialist Input Recommended)
- Pre-existing hepatic disease — given the risk of fulminant liver failure, PTU should be avoided in patients with active liver disease; if no alternative exists, a documented risk-benefit discussion and hepatology involvement are essential
- Pre-existing myelosuppression or bone marrow disorder — increased risk of agranulocytosis; baseline and ongoing haematological assessment required
- Concurrent use of other drugs associated with agranulocytosis (e.g., clozapine, carbamazepine) — additive myelotoxicity risk necessitates specialist monitoring
- Paediatric patients — PTU is not recommended for children except when methimazole cannot be used and surgery/RAI are not options, owing to a higher hepatotoxicity risk with no reported cases of methimazole-induced liver failure in paediatric patients
Use with Caution
- Elderly patients — limited data; start at lower end of dose range; agranulocytosis may be more frequent in patients over 40
- Pregnancy (second and third trimester) — switch to methimazole after the first trimester to avoid maternal hepatotoxicity risk; use the lowest effective dose throughout
- Patients on anticoagulants — intrinsic anti-vitamin K effect of PTU can potentiate bleeding
Cases of severe liver injury, including hepatic failure requiring liver transplantation or resulting in death, have been reported in both adult and paediatric patients treated with propylthiouracil. No cases of hepatic failure have been reported with methimazole in paediatric patients.
Propylthiouracil should be reserved for patients who cannot tolerate methimazole and in whom radioactive iodine therapy or surgery are not appropriate treatments. PTU may be the preferred antithyroid drug during or just prior to the first trimester of pregnancy due to the teratogenic risk associated with methimazole.
Biochemical monitoring of liver function is not expected to prevent severe liver injury owing to its rapid and unpredictable onset. Patients must be counselled to report symptoms of hepatic dysfunction immediately (FDA, April 2010).
Patient Counselling
Purpose of Therapy
Propylthiouracil reduces the amount of thyroid hormone your body produces. It is used to control the symptoms of an overactive thyroid gland when other treatments are not suitable. The medication does not work immediately — it may take several weeks before you start feeling better, and full control of thyroid levels typically takes two to four months.
How to Take
Take propylthiouracil three times daily at approximately eight-hour intervals, with or without food. Swallow the tablet whole with water. Do not stop taking the medication or change your dose without consulting your prescriber, even if you feel well. Missing doses can cause a rebound in thyroid hormone levels.
Sources
- Propylthiouracil Tablets, USP — FDA-approved prescribing information (NDA 006188). accessdata.fda.gov Primary regulatory source for approved indications, dosing, contraindications, and adverse reactions.
- FDA Drug Safety Communication: New Boxed Warning on severe liver injury with propylthiouracil. U.S. Food and Drug Administration. April 21, 2010. fda.gov Documents the postmarketing safety review that led to the boxed warning, including 34 cases of severe liver injury (13 deaths, 5 transplants).
- Nakamura H, Noh JY, Itoh K, et al. Comparison of methimazole and propylthiouracil in patients with hyperthyroidism caused by Graves’ disease. J Clin Endocrinol Metab. 2007;92(6):2157–2162. doi:10.1210/jc.2006-2135 RCT comparing adverse event rates between PTU and methimazole; showed significantly higher hepatotoxicity and overall adverse events with PTU.
- Noh JY, Inoue K, Suzuki N, et al. Dose-dependent incidence of agranulocytosis in patients treated with methimazole and propylthiouracil. Endocr J. 2024;71(7):711–718. doi:10.1507/endocrj.EJ24-0135 Large retrospective cohort (n=27,784) confirming dose-dependent agranulocytosis incidence with PTU (0.81% at 300 mg/day).
- Ross DS, Burch HB, Cooper DS, et al. 2016 American Thyroid Association Guidelines for Diagnosis and Management of Hyperthyroidism and Other Causes of Thyrotoxicosis. Thyroid. 2016;26(10):1343–1421. doi:10.1089/thy.2016.0229 Definitive guideline positioning methimazole as first-line ATD and reserving PTU for first-trimester pregnancy, thyroid storm, and methimazole intolerance.
- Alexander EK, Pearce EN, Brent GA, et al. 2017 Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and the Postpartum. Thyroid. 2017;27(3):315–389. doi:10.1089/thy.2016.0457 Provides trimester-specific antithyroid drug recommendations; supports PTU in first trimester with switch to methimazole thereafter.
- Kahaly GJ, Bartalena L, Hegedüs L, et al. 2018 European Thyroid Association Guideline for the Management of Graves’ Hyperthyroidism. Eur Thyroid J. 2018;7(4):167–186. doi:10.1159/000490384 European perspective on ATD management of Graves’ disease, aligning with ATA on methimazole preference and PTU’s role in pregnancy.
- Cooper DS. Antithyroid drugs. N Engl J Med. 2005;352(9):905–917. doi:10.1056/NEJMra042972 Comprehensive review of thioamide pharmacology including PTU’s dual mechanism (TPO inhibition + peripheral deiodinase blockade).
- Burch HB, Cooper DS. Anniversary Review: Antithyroid drug therapy: 70 years later. Eur J Endocrinol. 2018;179(5):R261–R274. doi:10.1530/EJE-18-0678 Historical and mechanistic review covering seven decades of ATD use, including PTU-specific hepatotoxicity pathophysiology.
- Kampmann JP, Hansen JM. Clinical pharmacokinetics of antithyroid drugs. Clin Pharmacokinet. 1981;6(6):401–428. doi:10.2165/00003088-198106060-00001 Foundational PK study establishing PTU half-life (1–2 h), bioavailability (80–95%), protein binding (~80%), and volume of distribution (~30 L).
- Amisha F, Rehman A. Propylthiouracil. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2023. NCBI Bookshelf Continuously updated clinical reference summarising PTU pharmacology, dosing, adverse effects, and monitoring for healthcare professionals.
- Rivkees SA. Controversies in the management of Graves’ disease in children. J Endocrinol Invest. 2016;39(11):1247–1257. doi:10.1007/s40618-016-0477-x Reviews the paediatric hepatotoxicity data that led to FDA restricting PTU use in children.