Pharmacologic class (EPC) · 2 drug pages

Thyroid hormone synthesis inhibitor: adverse event reports filed with FDA

3,734 reports list a drug in this class as a suspect or interacting product, 2004–2026.

Read first · FDA's statement on these data

Adverse event reports submitted to FDA do not undergo extensive validation or verification. Therefore, a causal relationship cannot be established between product and reactions listed in a report. While a suspected relationship may exist, it is not medically validated and should not be the sole source of information for clinical decision making or other assumptions about the safety or efficacy of a product. Additionally, it is important to remember that adverse event reports represent a small percentage of total usage numbers of a product. Common products may have a higher number of adverse events due to the higher total number of people using the product. openFDA, FDA

FDA adds: "There is no certainty that the reported event (adverse event or medication error) was actually due to the product. FDA does not require that a causal relationship between a product and event be proven, and reports do not always contain enough detail to properly evaluate an event." The information in these reports "cannot be used to estimate the incidence of these events."

Do not rely on openFDA to make decisions regarding medical care. While we make every effort to ensure that data is accurate, you should assume all results are unvalidated.

Not medical advice. This site counts what was reported to FDA. Questions about a medicine or device belong with a pharmacist or clinician.

3,734
reports across the class: 0% of all reports; 351
reports, 12 months to June 2026: 490 in the 12 months before; 87.2%
marked serious by the reporter: 57.4% across all reports; 2
drugs with a page: brand and generic names counted separately

Thyroid hormone synthesis inhibitor is a pharmacologic class (an FDA Established Pharmacologic Class) with 2 drug pages on this site. Together those drugs are listed as a suspect or interacting product in 3,734 adverse event reports received by FDA, 0% of the database, from January 2004 to June 2026.

351 reports arrived in the 12 months to June 2026, down 28% from 490 in the 12 months before. FDA releases FAERS data quarterly, so the newest months are incomplete and grow in later releases. 87.2% of the class's reports are marked serious, 7.7% record death among the outcomes and 45.1% record hospitalisation, as reported.

Across the class the reactions recorded most often are drug ineffective (11.9%), agranulocytosis (8.5%) and hyperthyroidism (6.8%). The class median share shown beside each term is the middle value across the member drugs; it is the comparison used on every drug page in this class.

The members table is ordered by report count for navigation only. FDA states that adverse event reports "represent a small percentage of total usage numbers of a product", that "common products may have a higher number of adverse events due to the higher total number of people using the product", and that the data "by themselves are not an indicator of the safety profile of the drug or biologic."

Drugs in the class

Drug	Name type	Reports	Latest 12 months	Marked serious	Top term recorded
Methimazole	generic	2,516	240	84.1%	Drug ineffective
Propylthiouracil	generic	1,218	111	93.6%	Drug ineffective

Ordered by report count for navigation only. FDA states that adverse event reports "represent a small percentage of total usage numbers of a product", that "common products may have a higher number of adverse events due to the higher total number of people using the product", and that the data "by themselves are not an indicator of the safety profile of the drug or biologic."

Reports by month, five years

FDA releases FAERS data quarterly, so the newest months are incomplete and grow in later releases.

Reactions recorded across the class

share of class reports recording the termmedian drug in the class

Drug ineffectivethe medicine did not work as expected11.9%445
Agranulocytosis8.5%316
Hyperthyroidismoveractive thyroid6.8%253
Foetal exposure during pregnancythe unborn baby was exposed to the medicine5.5%206
Off label useused for a purpose or in a way not on the label5%185
Exposure during pregnancythe medicine was taken during pregnancy3.5%132
Maternal exposure during pregnancythe mother took the medicine during pregnancy3.1%116
Thyrotoxic crisis3.1%115
Neutropenialow neutrophils, a type of white blood cell3%112
Anti-neutrophil cytoplasmic antibody positive vasculitis2.9%109
Drug-induced liver injury2.9%109
Pyrexiafever2.8%103
Hypothyroidismunderactive thyroid2.7%102
Premature babya baby born early2.5%93
Condition aggravatedthe condition being treated got worse2.4%88
Rashskin rash2.2%81
Product use in unapproved indicationused for a purpose not on the label2.1%79
Product use issuea problem in how the product was used1.9%71
Pruritusitching1.8%66
Arthralgiajoint pain1.7%65
Treatment noncompliancethe treatment was not taken as directed1.7%64
Nauseafeeling sick1.6%61
Acute hepatic failure1.5%56
Pancytopenialow counts of all blood cells1.5%55
Anaemialow red blood cells1.3%50

Solid bar: share of all class reports. Hollow bar: the median share across the 2 member drugs, the reference used on each member's page. One report can record several terms.

Outcomes, patients and reporters

Outcomes recorded

Death7.7%288

Life-threatening11.6%434

Hospitalisation (initial or prolonged)45.1%1,683

Disability0.7%26

Congenital anomaly4.3%160

Other serious56.5%2,111

Not serious12.8%478

Patient age

Under 21.2%46

2 to 111.9%70

12 to 174%148

18 to 4429.4%1,099

45 to 6422%823

65 to 748.5%318

75 and over6.7%252

Age not given26.2%978

Who reported

Physician28%1,045

Pharmacist4.7%177

Other health professional51%1,905

Lawyer0.1%2

Consumer or non-health professional12.1%450

Not given4.2%155

Grey bar: all 20,646,523 reports in the database. Outcome flags are as the reporter set them and are not verified by FDA.

Questions about Thyroid hormone synthesis inhibitor reports

Which drugs are in the Thyroid hormone synthesis inhibitor class on this site?

Methimazole and Propylthiouracil. Class membership comes from the pharmacologic class (EPC) on the drug's FDA label.

How many reports does the class have?

3,734 reports through June 2026, 351 of them in the latest 12 months. A report listing two drugs of the class counts once for each.

What reactions are recorded most often?

drug ineffective (11.9%), agranulocytosis (8.5%), hyperthyroidism (6.8%), foetal exposure during pregnancy (5.5%) and off label use (5%).

Can the members table be used to rank the drugs?

No. FDA states that adverse event reports "represent a small percentage of total usage numbers of a product", that "common products may have a higher number of adverse events due to the higher total number of people using the product", and that the data "by themselves are not an indicator of the safety profile of the drug or biologic." Drugs differ in how many people take them, in the conditions they treat, in time on the market and in how diligently reports are collected.

How do I report an adverse event?

Through FDA MedWatch (Form FDA 3500, or 1-800-FDA-1088) or to the manufacturer. See the guide on how to report.

Where this comes from. Counts are built from the openFDA Drug Adverse Event (FAERS) bulk export released 28 Sep 2026, which holds reports FDA received through 30 Jun 2026. Drug names are as reported, resolved to the brand or generic name on the FDA label where openFDA's harmonisation matched them; classes are the pharmacologic class (EPC) from the label and the National Drug Code Directory of 17 Sep 2026. A class is the Established Pharmacologic Class most often attached to a drug's reports by openFDA's label harmonisation, or from the NDC Directory where reports carry none. No report narrative, lot number, patient detail or reporter identity is published. See the methodology and sources; FDA's record is authoritative, and corrections are handled within five working days.

Drug data (FAERS) through 30 Jun 2026; device data (MAUDE) through 31 Aug 2026. Not medical advice.