Thyroperoxidase (inhibitor, no IC₅₀/Ki/EC₅₀ values provided in this paper) [1].

In human beta cells, methimazole TNF for CXC chemokine ligand 10 [3].

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In Vitro: In human thyrocytes isolated from multinodular goiter patients, Methimazole (1–1000 ng/ml, 24 h) dose‑dependently decreased CXCL10 secretion induced by IFNγ (1000 U/ml) + TNFα (10 ng/ml), with maximum inhibition of 46.28 ± 3.04% at 300 ng/ml (2.63 μM). IC₅₀ (ALLFIT interpolation) was 0.29 ± 0.05 μM, comparable to rosiglitazone [2].
Methimazole (300 ng/ml) significantly reduced TNFα‑induced IFNγR membrane protein expression (from 30 ± 1.5% to 17.4 ± 3.8% positive cells) and IFNγR mRNA in thyrocytes, as shown by flow cytometry and real‑time PCR [2].
In isolated rat hepatocytes, Methimazole (10 mM, 120 min) caused ~50% cell death (LC₅₀). In GSH‑depleted hepatocytes (1‑bromoheptane pretreatment), the LC₅₀ dropped to 5 μM [3].
Methimazole (10 mM) induced collapse of mitochondrial membrane potential (MMP) as measured by rhodamine 123 fluorescence, increased reactive oxygen species (ROS) formation (DCFH‑DA assay), and increased lipid peroxidation (TBARS). These effects were more severe in GSH‑depleted cells and were prevented by glyoxal trapping agents (metformin, NAC, hydralazine) or the CYP inhibitor cobalt chloride [3].
Methimazole (10 mM) caused a significant reduction in cellular glutathione (GSH) content in rat hepatocytes [3].
N‑methylthiourea (1 mM), a proposed methimazole metabolite, induced cell death and GSH depletion but did not cause ROS formation, MMP collapse, or lipid peroxidation. Its toxicity was prevented by the FMO inhibitor N,N‑dimethylaniline (DMA) [3].

In vivo, male Wistar rats orally treated with Methimazole (0.05% water solution ad libitum for 90 days) showed a significant 30% decrease in final body weight (418.8±19.0 g vs. 598.7±30.9 g in controls, P=0.01). Serum osteocalcin (OC, bone formation marker) was significantly decreased by 21% (3.06±0.46 ng/mL vs. 3.89±0.24 ng/mL, P=0.02), while serum C-terminal telopeptides of type I collagen (CTX-I, bone resorption marker) showed a strong tendency to increase by 17% (0.056±0.003 ng/mL vs. 0.048±0.002 ng/mL, P=0.06) [1].
Methimazole treatment significantly decreased morphological, geometric and densitometric parameters of femur and tibia: bone weight, bone length, bone volume, mean volumetric bone mineral density (MvBMD), bone mineral density (BMD), cross-sectional area (tibia only), second moment of inertia, trabecular bone area, cortical bone area, periosteal circumference, axial and polar moments of inertia, polar moment of resistance, and strength-strain index (SSI) were all significantly reduced (P<0.01). For femur, bone mineral content (BMC), cortical bone mineral density, maximum elastic strength (Wy) and ultimate strength (Wf) were significantly decreased (P<0.05). Maximum elastic strength and ultimate strength of femur were reduced by 35.9% and 40.4%, respectively (Wy: 77±4 N vs. 120±22 N; Wf: 90±5 N vs. 151±20 N, P<0.05). Tibia mechanical properties (Wy, Wf) were not significantly different [1].

Enzyme Assay: No direct enzyme kinetic assays were performed. CYP inhibition: In isolated rat hepatocytes, cobalt chloride (250 μM, 30 min preincubation) – a nonspecific CYP450 inhibitor – significantly reduced Methimazole‑induced cell death, ROS formation, MMP collapse, and lipid peroxidation. Cimetidine (2 mM) had no significant effect [3].
FMO inhibition: The flavin‑containing monooxygenase inhibitor N,N‑dimethylaniline (DMA, 200 μM, 30 min preincubation) attenuated N‑methylthiourea‑induced cytotoxicity but did not significantly reduce methimazole‑induced cell death [3].
Glyoxal trapping assay: Co‑incubation with metformin (5 mM), N‑acetylcysteine (NAC, 200 μM), or hydralazine (300 μM) effectively prevented methimazole‑induced cytotoxicity, ROS formation, MMP collapse, and lipid peroxidation in intact and GSH‑depleted hepatocytes [3].

Cell Assay: Primary human thyrocyte culture: Thyrocytes were isolated from intermodular parenchyma of multinodular goiter patients (n=15). Cells were cultured in DMEM/Ham’s F‑12 (1:1) with 10% FBS, 2 mM glutamine, antibiotics. For CXCL10 secretion, 4000 cells/well in 96‑well plates were serum‑starved, then treated with IFNγ (1000 U/ml) and TNFα (10 ng/ml) ± Methimazole (1–1000 ng/ml) or rosiglitazone for 24 h. CXCL10 in supernatant was measured by ELISA (sensitivity 1.67 pg/ml). Protein was extracted with 1 M NaOH and quantified by Bradford assay. Results normalized to protein content [2].
Flow cytometry for IFNγR (CD119): Thyrocytes (10⁵ cells) were stained with PE‑conjugated anti‑CD119 or isotype control, analyzed on a BD LSR II. Methimazole (300 ng/ml, 24 h) reduced TNFα‑induced IFNγR expression [2].
Real‑time PCR: RNA extracted with RNeasy kit, reverse transcribed, and Taqman qPCR performed for CXCL10, IFNγR, and Pax8 (thyroid marker) on an ABI Prism 7700. Data normalized to 18S rRNA, expressed as 2⁻ΔΔCt [2].
Immunofluorescence for NF‑κB p65: Thyrocytes on coverslips were treated with TNFα±IFNγ ± MMI or RGZ, fixed, permeabilized, stained with anti‑p65 primary and Alexa Fluor 488 secondary, and examined by microscopy. MMI did not inhibit nuclear translocation of NF‑κB [2].
Isolated rat hepatocyte cytotoxicity assay: Hepatocytes (10⁶ cells/ml) from male Sprague‑Dawley rats were prepared by collagenase perfusion. Cells incubated in Krebs‑Henseleit buffer (pH 7.4, 37 °C, 95% O₂/5% CO₂) in rotating flasks. Viability assessed by trypan blue (0.1% w/v) exclusion. LC₅₀ for Methimazole was determined after 120 min [3].
MMP assay: Hepatocytes (2 ml samples) were centrifuged, resuspended in buffer containing 1.5 μM rhodamine 123, incubated 37 °C, then centrifuged. Fluorescence of supernatant measured at 490 nm excitation/520 nm emission. Decreased fluorescence indicates MMP collapse [3].
ROS formation: Hepatocytes incubated with 1.6 μM DCFH‑DA. Fluorescence measured at 500 nm excitation/520 nm emission. Increased fluorescence indicates ROS [3].
Lipid peroxidation (TBARS): 1 ml hepatocyte suspension treated with 250 μl 70% TCA, centrifuged. Supernatant mixed with 0.8% TBA, boiled 20 min, absorbance read at 532 nm [3].
GSH content: Hepatocyte samples precipitated with 5% TCA, then reacted with Ellman’s reagent (DTNB) and phosphate buffer (pH 8.0), absorbance at 412 nm. Samples reduced with KBH₄ to prevent autoxidation [3].
GSH depletion: Hepatocytes pretreated with 1‑bromoheptane (200 μM, 30 min) before methimazole exposure [3].
Enzyme inhibition: Cobalt chloride (250 μM), cimetidine (2 mM), or DMA (200 μM) added 30 min before methimazole [3].
Glyoxal trapping: Metformin (5 mM), NAC (200 μM), or hydralazine (300 μM) co‑incubated with methimazole [3].

Animal protocol: Male Wistar rats (initial body weight 220‑260 g, 9 weeks of age at start of treatment) were divided into control (N=6) and experimental (N=6) groups. All animals were kept under identical environmental conditions (air‑conditioned vivarium, 22‑23 °C, 45‑47% humidity, 12/12 h light/dark cycle) with free access to a standard diet fed ad libitum. The experimental group received tap water containing 0.05% Methimazole (water solution prepared fresh daily) ad libitum for 90 days. The control group received plain tap water. After 90 days, rats were anaesthetized with ketamine (80 mg/kg i.m.), blood was collected by cardiac puncture, and left and right femur and tibia were isolated postmortem for further analysis [1].

Absorption, Distribution and Excretion
Methimazole is rapidly and extensively absorbed after oral administration, with an absolute bioavailability of approximately 0.93 and a time to peak concentration (Tmax) ranging from 0.25 to 4.0 hours. Peak plasma concentrations (Cmax) are slightly elevated in patients with hyperthyroidism, but the difference is not statistically significant. Both Cmax and AUC are significantly affected by the oral dose. The urinary excretion rate of unmetabolized methimazole is reported to be 7% to 12%. Fecal excretion appears to be limited, with a cumulative fecal excretion rate of 3% after methimazole administration. Enterohepatic circulation also appears to be involved in the clearance of methimazole and its metabolites, as significant amounts of these substances have been detected in bile after administration. The apparent volume of distribution of methimazole is reported to be approximately 20 liters. After oral administration, methimazole is highly concentrated in the thyroid gland—the concentration of methimazole in the thyroid is approximately 2 to 5 times the peak plasma concentration, and remains at a high level for up to 20 hours after ingestion.
After a single intravenous bolus injection of 10 mg methimazole, the clearance rate was 5.70 L/h. Renal impairment did not appear to affect the clearance rate of methimazole, but the clearance rate decreased in patients with hepatic impairment in roughly proportional to the severity of hepatic impairment—moderate hepatic impairment resulted in a clearance rate of 3.49 L/h, while severe hepatic impairment resulted in a clearance rate of 0.83 L/h. There appeared to be no significant difference in clearance rate based on thyroid status (i.e., between patients with euthyroid and hyperthyroidism).
Four days after intravenous injection of 14C-methimazole in rats, the highest retention of 14C was observed in the thyroid and adrenal glands; 76% of the dose was excreted in the urine and 6% in the feces.
The radioactivity of 14C-methimazole was higher in the thyroid than in any other tissue, with a thyroid/plasma ratio reaching 62.5 after four consecutive days of administration.
Oral administration to rats resulted in complete absorption. ...Binding to plasma proteins is negligible, and...even after intravenous injection, it exhibits unicompartmental kinetics. ...This is attributed to the high lipid-water partition coefficient of methimazole, which allows for faster tissue penetration.
The amount of radioactive material excreted from...(35)S-methimazole...in bile is only 21% of the intravenously administered dose. Biliary radioactivity is almost entirely derived from metabolites...
For more complete data on the absorption, distribution, and excretion of methimazole (9 types), please visit the HSDB record page.

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Metabolic/Metabolic Substances
Methimazole is rapidly and extensively metabolized in the liver primarily through the CYP450 and FMO enzyme systems. Although several metabolites have been identified, the specific isoenzymes responsible for their formation are not fully understood. One of the earliest identified methimazole metabolites, 3-methyl-2-thiohydantoin, may possess antithyroid activity—this activity has been confirmed in rats, which may explain why iodination inhibition persists for a relatively long time after administration, despite methimazole's relatively short half-life. Many metabolites have been investigated and are considered major contributors to methimazole-induced hepatotoxicity. Glyoxal and N-methylthiourea are both known cytotoxic and are known metabolites of the methimazole dihydrodiol intermediate. Methimazole's sulfinic acid and sulfonic acid derivatives are considered the final toxicants leading to hepatotoxicity, but their origin is unclear—they may arise from direct oxidation of methimazole via FMO or from the oxidation of downstream N-methylthiourea during metabolism. In Sprague-Dawley rats, up to 21% of the dose was excreted unchanged in the urine over 24 hours, with glucuronide being the major metabolite (36-48%); the remaining urinary metabolites have not been identified.
Methimazole, upon incubation with rat liver microsomes, produces 3-methyl-2-thiohydantoin and N-methylimidazole.
Biological Half-Life
Following a single intravenous bolus injection of 10 mg methimazole, the half-life in the distribution phase is 0.17 hours, and the half-life in the elimination phase is 5.3 hours. The half-life of 3-methyl-2-thiohydantoin, the major active metabolite of methimazole, is approximately three times that of its parent drug. Renal impairment does not appear to alter the half-life of methimazole, but the half-life is prolonged in patients with hepatic impairment, with the degree of prolongation roughly proportional to the severity of hepatic impairment—moderate hepatic impairment results in an elimination half-life of 7.1 hours, while severe hepatic impairment results in an elimination half-life of 22.1 hours. Thyroid function status does not appear to have a significant effect on the half-life (i.e., there is no difference in the half-life between patients with euthyroid and hyperthyroidism).
Plasma half-life is 3–5 hours. The elimination half-life of methimazole is reported to be approximately 5-13 hours. The plasma half-life of methimazole is approximately 4-6 hours.

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ADME/Pharmacokinetics: In humans, after oral ingestion of 40 mg Methimazole, maximum serum concentration (Cₘₐₓ) averages 0.542 μg/ml (4.75 μM), reached at tₘₐₓ of 60 min. This concentration is close to the LC₅₀ observed in GSH‑depleted rat hepatocytes (5 μM) [3].
In the thyrocyte study, Methimazole concentrations used (300 ng/ml = 2.63 μM) were selected based on near‑therapy doses according to pharmacokinetic parameters (Cₘₐₓ and AUC) [2].
Methimazole is metabolized via cytochrome P450 (CYP) enzymes and flavin‑containing monooxygenase (FMO). Metabolites include N‑methylthiourea and glyoxal [3].
In human thyrocytes, MMI reduces TNFα‑induced IFNγR expression, which is an immunomodulatory effect not directly related to metabolism [2].

Interactions
Pentobarbital can increase bile excretion. The combined use of propylthiouracil and methimazole can lead to an increased serum triiodothyronine (T3) to thyroxine (T4) ratio in patients, possibly due to direct drug action on the thyroid gland or peripheral T4 deiodination to T3. The metabolic clearance of aminophylline and theophylline is increased in hyperthyroid patients, but this clearance returns to normal as thyroid function normalizes; when thyroid function returns to normal, the dosage of aminophylline, hydroxypropyltheophylline, or theophylline may need to be reduced. /Antithyroid Drugs/ Iodides or iodine excess may reduce the efficacy of antithyroid drugs, requiring increased dosage or prolonged treatment; amiodarone contains 37% iodine (by weight), therefore its use significantly increases iodine intake; iodine deficiency may enhance the efficacy of antithyroid drugs, thus requiring reduced dosage or shortened treatment. /Antithyroid Drugs/
As the patient's thyroid and metabolic status gradually return to normal, the efficacy of oral anticoagulants may decrease; however, if hypoprothrombinemia caused by thioamide drugs occurs, the anticoagulant effect may be enhanced; it is recommended to adjust the dosage of oral anticoagulants according to prothrombin time. /Antithyroid Drugs/
For more interaction (complete) data on methimazole (6 types in total), please visit the HSDB record page.

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Non-human toxicity values
Rats oral LD50: 2250 mg/kg
Rats subcutaneous LD50: 1050 mg/kg
Mice oral LD50: 860 mg/kg
Mice intraperitoneal LD50: 500 mg/kg
Mice subcutaneous LD50: 345 mg/kg

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Toxicity/Toxicokinetics: In this study, long‑term oral Methimazole treatment caused significant reduction in body weight gain (30% lower final body weight) and negative effects on bone tissue metabolism, morphology, density and mechanical strength. The authors discuss that Methimazole overdosing or inappropriate monitoring may result in hypothyreosis with similar metabolic effects. Previous reports (cited in the discussion) have shown that Methimazole can cause acute toxic hepatitis, liver injury, cholestatic liver injury, and increased risk for hepatitis and acute liver failure. Possible mechanisms include reactive metabolite formation, oxidative stress induction, disruption of calcium homeostasis, immune‑mediated toxicity, and deactivation of antioxidant enzymes. No specific LD₅₀ or quantitative toxicity data are provided in this paper [1].

[1]. Effects of long-term oral administration of methimazole on femur and tibia properties in male Wistar rats. Biomed Pharmacother. 2017 Oct;94:124-128.

[2]. Mechanisms of methimazole cytotoxicity in isolated rat hepatocytes. Drug Chem Toxicol. 2013 Oct;36(4):403-11.

[3]. Methimazole inhibits CXC chemokine ligand 10 secretion in human thyrocytes. J Endocrinol. 2007 Oct;195(1):145-55.

[4]. Comparison of methimazole and propylthiouracil in patients with hyperthyroidism caused by Graves' disease. J Clin Endocrinol Metab. 2007 Jun;92(6):2157-62. Epub 2007 Mar 27.

Therapeutic Uses

Antithyroid Drugs
Methimazole is approximately 10 times more potent than propylthiouracil and induces an antithyroid response more rapidly. Its effects are also more prolonged than propylthiouracil…
…It is used to treat hyperthyroidism…(1) as a radical treatment to control the condition in the hope of observing spontaneous remission in Graves' disease;(2) in combination with radioactive iodine to accelerate recovery while awaiting the effects of radiotherapy; and(3) to control the condition during preoperative preparation. /Antithyroid Drugs/
Currently, there are no commercially available injectable formulations for the rare cases where oral administration is not possible. For such cases, and for experimental purposes, the water-soluble compound methimazole can be dissolved in physiological saline and sterilized by heat.
Methimazole is indicated for the treatment of hyperthyroidism, including before surgery or radiotherapy, and as an adjunct therapy for thyrotoxicosis or thyroid storm. In the treatment of thyroid storm, propylthiouracil may be superior to methimazole because propylthiouracil inhibits the peripheral conversion of thyroxine (T4) to triiodothyronine (T3). /US product label contains/

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Drug Warnings
…Women taking these medications should not breastfeed their infants. /Antithyroid Drugs/
Regular white blood cell counts are not very helpful because agranulocytosis can develop rapidly. Patients should report symptoms such as sore throat or fever immediately, as these are often precursors to the reaction.
The main disadvantage of antithyroid drug treatment is the high relapse rate after discontinuation. Long-term, frequent medication is another disadvantage, although adverse reactions are uncommon and rarely severe, they are still a disadvantage.
Susceptible patients may have cross-sensitivity reactions to other thiamine derivatives.
For more complete data on drug warnings for methimazole (13 in total), please visit the HSDB record page.

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Pharmacodynamics
Methimazole inhibits the synthesis of thyroid hormones, thereby relieving hyperthyroidism. The onset of action is 12 to 18 hours, and the duration of action is 36 to 72 hours, likely due to the concentration of methimazole and some of its metabolites in the thyroid gland after administration. The most serious potential side effect of methimazole treatment is agranulocytosis; patients should be instructed to monitor for and report any signs or symptoms of agranulocytosis, such as fever or sore throat. Other cytopenias may also occur during methimazole treatment. Furthermore, there is a risk of serious hepatotoxicity with methimazole; therefore, patients receiving this therapy should be carefully monitored for signs and symptoms of liver dysfunction, such as jaundice, anorexia, pruritus, and elevated liver transaminases.

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Methimazole is an antithyroid drug that inhibits thyroperoxidase, reducing thyroid hormone biosynthesis. This study demonstrates that long‑term oral administration of Methimazole in rats leads to inhibition of bone formation (decreased serum osteocalcin) and accelerated bone resorption (increased CTX‑I), resulting in impaired skeletal properties. These skeletal side effects are considered additional possible adverse effects beyond the known hepatotoxicity. The observed negative consequences on body weight gain and bone tissue may be related to methimazole‑induced liver impairments, as the liver plays a crucial role in vitamin D and calcium metabolism, vitamin K synthesis, osteocalcin carboxylation, and IGF‑1 production [1].

C4H6N2S

114.1688

114.025

60-56-0

Methimazole-d3;1160932-07-9

1349907

White to off-white solid powder

1.3±0.1 g/cm3

280.0±9.0 °C at 760 mmHg

144-147 °C(lit.)

123.1±18.7 °C

0.0±0.6 mmHg at 25°C

1.633

-0.34

1

1

0

7

119

0

PMRYVIKBURPHAH-UHFFFAOYSA-N

InChI=1S/C4H6N2S/c1-6-3-2-5-4(6)7/h2-3H,1H3,(H,5,7)

3-methyl-1H-imidazole-2-thione

2934.99.9001

Powder      -20°C    3 years

                     4°C     2 years

In solvent   -80°C    6 months

                  -20°C    1 month

Room temperature (This product is stable at ambient temperature for a few days during ordinary shipping and time spent in Customs)

DMSO : ≥ 100 mg/mL (~875.89 mM)
H2O : ≥ 50 mg/mL (~437.94 mM)

Solubility in Formulation 1: ≥ 2.5 mg/mL (21.90 mM) (saturation unknown) in 10% DMSO + 40% PEG300 + 5% Tween80 + 45% Saline (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 25.0 mg/mL clear DMSO stock solution to 400 μL PEG300 and mix evenly; then add 50 μL Tween-80 to the above solution and mix evenly; then add 450 μL normal saline to adjust the volume to 1 mL.
Preparation of saline: Dissolve 0.9 g of sodium chloride in 100 mL ddH₂ O to obtain a clear solution.

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Solubility in Formulation 2: ≥ 2.5 mg/mL (21.90 mM) (saturation unknown) in 10% DMSO + 90% (20% SBE-β-CD in Saline) (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 25.0 mg/mL clear DMSO stock solution to 900 μL of 20% SBE-β-CD physiological saline solution and mix evenly.
Preparation of 20% SBE-β-CD in Saline (4°C，1 week): Dissolve 2 g SBE-β-CD in 10 mL saline to obtain a clear solution.

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Solubility in Formulation 3: ≥ 2.5 mg/mL (21.90 mM) (saturation unknown) in 10% DMSO + 90% Corn Oil (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 25.0 mg/mL clear DMSO stock solution to 900 μL of corn oil and mix evenly.

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Solubility in Formulation 4: 100 mg/mL (875.89 mM) in PBS (add these co-solvents sequentially from left to right, and one by one), clear solution; with ultrasonication.

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 (Please use freshly prepared in vivo formulations for optimal results.)