Determination of propylthiouracil and methylthiouracil in human serum using high-performance liquid chromatography with chemiluminescence detection

Based on the sensitizing effect of formaldehyde on the chemiluminescence (CL) reaction of propylthiouracil (PTU) and methylthiouracil (MTU) with acidic potassium permanganate and the combination technique of high-performance liquid chromatography (HPLC), a sensitive, selective and simple post-column CL detection method for determining PTU and MTU is described. The optimal conditions for the CL detection and HPLC separation were carried out. The linear ranges were 0.1-20 microg mL(-1) for MTU and 0.1-10 microg mL(-1) for PTU, the detection limits were 0.03 microg mL(-1) for PTU, 0.03 microg mL(-1) for MTU and the quantification limits were 0.1 microg mL(-1) for PTU, 0.1 microg mL(-1) for MTU. The method has been satisfactorily applied for the determination of MTU and PTU in human serum samples.

Premature twins of a mother with Graves' disease with discordant thyroid function: a case report

Thyroid dysfunction is recognized in the newborns of mothers affected by Graves' disease during pregnancy. We describe the development of concurrent hyperthyroidism and hypothyroidism in the twin infants of a mother with Graves' disease diagnosed during pregnancy.

Therapeutic drug monitoring of antithyroid drugs in pregnancy: the knowledge gaps

Despite being a common condition in pregnancy, and despite propylthiouracil (PTU) being perceived as safer than methimazole, there are virtually no epidemiological controlled studies on malformation rate an neurobehavioral outcomes with the former. This knowledge gap must be filled to ensure fetal safety.

The effect of the antithyroid drug propylthiouracil on the alternative pathway of complement in rats

The effect of propylthiouracil (PTU) on the lytic activity of complement in rat serum was investigated in vivo. Rats (180+/-10 g) were treated daily by gavage with PTU doses of 1-50 mg/200 g body weight for time intervals ranging from 1 to 30 days. Serum classical pathway (CP) and alternative pathway (AP) activities were determined 24 h after the last dose. A single dose of 50 mg/200 g body weight was administered to additional groups and the animals were sacrificed after periods of 1-48 h. The results showed a relatively small reduction ( approximately 30%) in CP activity, evident only in animals treated with 50 mg of PTU for three weeks. However, a clear and opposite effect of PTU, an increase in lytic activity reaching values up to 180% of controls, was observed on AP activity. This effect was seen at all PTU doses used, and occurred within 4 days of treatment with the highest dose. Maximum activity was observed at intermediate intervals, depending on the PTU dose, with a return to control levels occurring after the longer periods of treatment. The lytic activity of serum from animals treated with a single PTU dose of 50 mg/200 g body weight and sacrificed 1-48 h after dosing did not differ from controls. Serum levels of thyroid hormone (triiodo L-thyronine, T3, and thyroxine, T4) were determined in representative groups of treated animals (injected with 5 mg of PTU/200 g body weight/day). These were either undetectable or considerably lower than those of controls. The serum PTU levels of these rats increased for up to 22 days, reaching values of 2-4 microg/ml.PTU is described in the literature as a modulator of both cellular immune responses and antibody production. Upon complement activation fragments of complement components bind to immune complexes and to specific receptors on cells of the immune system. Thus, alteration in AP activity caused by PTU treatment suggests a possible mechanism by which the drug exerts its modulatory effect. Increased complement AP activity might affect events as antigen presentation and hence the onset and course of the immune response.

Duodenal obstruction in thyroid storm

A 35-year-old, previously healthy woman, known to be thyrotoxic, was transferred from a community hospital for "acute abdomen." Abdominal pain, distention, and hyperemesis resolved with placement of nasogastric tube (NGT) and return of 2,600 mL of bilious fluid. Continued high NGT output made oral or NGT administration of antithyroid drugs impossible. We gave propylthiouracil (PTU) by retention enemas with therapeutic serum levels and sublingual saturated solution of potassium iodide (SSKI) with 70% absorption based on 24-hour free iodine urinary excretion. The patient's thyroxine (T4) and triiodothyronine (T3) radioimmunoassays were normal on hospital days 10 and 12, respectively. However, free T4 and T3 resin uptake did not normalize until hospital day 31. On hospital day 32, she tolerated removal of NGT without nausea and 4 days later was taking a regular diet. We conclude that our patient's gastrointestinal symptoms were a prominent feature of her thyrotoxicosis and that rectal PTU and sublingual SSKI are effective in administration of antithyroid drugs.

[The effects of propylthiouracil on the fetal thyroid and serum concentration in pregnant women]

In order to study the effects of propylthiouracil (PTU) on the fetal thyroid and serum concentration, 9 women undergoing therapeutic abortion at the second trimester of pregnancy were given a single dose of 50, 100 and 150 mg PTU respectively. The concentrations of PTU in the maternal and fetal serum and those in the fetal thyroid tissues were determined with a high-performance liquid chromatographic method, the lowest limit of which was 25 micrograms/L. After an oral bolus of PTU, the PTU concentrations in the maternal serum reached a peak level after one hour and there was a wide individual variation of the peak value. The study showed PTU could be transferred through the placenta from the maternal blood to the fetal blood and highly accumulated in the fetal thyroid tissues. The fetal serum PTU concentrations correlated with those of their mothers at the same time (r = 0.7084, P < 0.02) and the concentrations in the fetal thyroid tissues could be evaluated by the maternal serum peak level after an oral bolus. PTU didn't inhibit the fetal thyroid peroxidase activity even when the maternal serum PTU concentration reached 4 mg/L, which is the lowest effective therapeutic concentration of PTU in adults.

Modulation by thyroid hormones of the development of external plexiform layer in the rat olfactory bulb

Width of the external plexiform layer in olfactory bulbs and mean area of mitral and granule cell dendritic and glial processes were measured of normal, hypo- and hyperthyroid rat pups at the age of 24 days. Hypothyroidism was induced by treating the rats with a reversible goitrogen 6-n-propyl-2-thiouracil dissolved in their drinking water, while the hyperthyroid group was given water containing thyroxine. The 6-n-propyl-2-thiouracil treatment was begun on gestational day 18 and on the day of birth. Thyroxine treatment started on the day of birth. Both treatments were continued till the day of sacrifice. A significant decrease in the width of the external plexiform layer of the olfactory bulb in the prenatally 6-n-propyl-2-thiouracil treated group and a significant increase in the width of the external plexiform layer of the hyperthyroid group was shown by the Student's paired t-test. The areas of neuronal and glial processes were measured at electron microscopic level by using an IBAS image analysing system. A significant decrease was found by the Kruskal-Wallis test and Dunn's range test in the mean area of (1) mitral cell dendrites in the prenatal 6-n-propyl-2-thiouracil treated group, (2) granule cell dendrites in both the postnatally 6-n-propyl-2-thiouracil treated and in the hyperthyroid groups and (3) glial processes in the thyroxine treated group comparing to controls.

Vasodilatory effects of propylthiouracil in patients with alcoholic cirrhosis

BACKGROUND/AIMS

An experimental study has shown that propylthiouracil increases portal blood flow in normal rats. Whether propylthiouracil has a similar effect in patients with alcoholic cirrhosis remains to be demonstrated. The aim of this study was to evaluate the effects of oral propylthiouracil (300 mg) on systemic and portal hemodynamics in patients with alcoholic cirrhosis.

METHODS

Plasma propylthiouracil levels were also measured by high performance liquid chromatography in five patients with alcoholic cirrhosis. In eight patients with cirrhosis, mean arterial pressure, cardiac output and portal blood flow were evaluated before and after placebo and propylthiouracil administration. Hemodynamic measurements were performed by the Doppler technique. The plasma peak level of propylthiouracil was achieved at 1.4 +/- 0.1 h in patients with alcoholic cirrhosis. This time was chosen to express hemodynamic changes.

RESULTS

Propylthiouracil administration caused a significant increase in portal blood flow (+16.5%, p < 0.05) in patients with alcoholic cirrhosis. This effect was associated with a mild and significant rise in cardiac output (from 5.8 +/- 0.2 to 6.1 +/- 0.3 l/min, p < 0.05) and a decrease in peripheral vascular resistance (from 1171 +/- 69 to 1070 +/- 67 dyn . s-1 . cm-5, p < 0.01). A significant correlation was observed between changes in portal blood flow and peripheral vascular resistance (r = 0.79, p < 0.05). No significant changes were observed after placebo.

CONCLUSIONS

Our findings show that propylthiouracil has a vasodilatory effect in patients with alcoholic cirrhosis. We postulate that this effect could be the mechanism by which propylthiouracil decreases hypermetabolic state, and increases oxygen delivery to the liver, in patients with alcoholic liver diseases.

Selective liquid chromatographic assay for propylthiouracil in plasma

A liquid chromatographic assay was developed to quantitate propylthiouracil in plasma using an internal standard, 5-propyl-2-thiouracil, of similar structure and physical properties. Caffeine, which coelutes with propylthiouracil, was removed by extraction from serum treated with base. No other compounds were found to interfere in the assay. The drug was extracted from plasma with chloroform with a recovery of 59.4% and the intra- and inter-assay coefficients of variation were 5.7 and 3.3%, respectively. The assay was linear to 3 micrograms/ml with a lower detection limit of 40 ng/ml for a sample volume of 1 ml.

High-performance liquid chromatographic determination of propylthiouracil in plasma

A high-performance liquid chromatographic method for the determination of the antithyroid drug propylthiouracil in dog plasma has been developed. Propylthiouracil and the internal standard, methylthiouracil, are extracted from plasma with methylene chloride at pH 6 and the organic layer is evaporated to dryness. The residue is chromatographed on a Chromspher C18 reversed-phase column using Pic B-7 (0.005 M 1-heptanesulphonic acid in water)-1% acetic acid-methanol (40:45:15, v/v/v) as the mobile phase. Quantification is achieved by monitoring the UV absorbance at 300 nm. The response is linear (0.1-15 micrograms/ml) and using 100 microliters of plasma the detection limit is 50 ng/ml. The within-run coefficient of variation is less than or equal to 5% and the accuracy is within 10% of the theoretical value at concentrations between 0.1 and 15 micrograms/ml plasma.

The binding of thioureylene compounds to human serum albumin

The binding interactions of some thioureylene compounds to human serum albumin were studied in vitro by ultraviolet spectroscopy and equilibrium dialysis. Binding of 6-n-propyl-2-thiouracil, 6-n-benzyl-2-thiouracil and 2-thiouracil to human serum albumin results in a red shift of the ultraviolet absorption maximum, suggesting that the binding site is a hydrophobic area of the protein. Bindings of 6-n-propyl-2-thiouracil and 6-n-benzyl-2-thiouracil to human serum albumin are characterized by two classes of sites while 6-n-propyl-uracil and 2-thiouracil bind to one low-affinity binding site. In addition, an identification of those sites was performed by measuring the displacement of these drugs. The data show that the moderate-affinity site is common with the warfarin site while the low-affinity site is likely to be shared by benzodiazepines. It is concluded that the binding is enhanced by the hydrophobicity of the substituent in the thioureylene compounds, and it is further shown that thiol-group substitutions in the thioureylene ring will weaken the binding.

Altered disposition of propylthiouracil in cats with hyperthyroidism

The oral and intravenous disposition of the anti-thyroid drug propylthiouracil (PTU) was determined in six clinically healthy cats and four cats with naturally occurring hyperthyroidism. Compared with the normal cats, the mean plasma elimination half-life of PTU was significantly (P less than 0.001) shorter in the hyperthyroid cats (77.5 +/- 5.8 minutes compared with 125.5 +/- 3.7 minutes) and the total body clearance of PTU was significantly (P less than 0.05) more rapid in the cats with hyperthyroidism (5.1 +/- 0.8 ml kg-1 min-1 compared with 2.7 +/- 0.2 ml kg-1 min-1). Following oral administration, both the bioavailability (59.7 +/- 4.9 per cent compared with 73.3 +/- 3.7 per cent) and peak plasma concentrations (14.5 +/- 1.6 micrograms ml-1 compared with 18.9 +/- 0.9 micrograms ml-1) of PTU were significantly (P less than 0.05) lower in the hyperthyroid cats than in the control cats. No difference was noted, however, between the apparent volume of distribution for PTU in the two groups of cats. Overall, results of this study indicate that the oral bioavailability of PTU is decreased and PTU disposition is accelerated in cats with hyperthyroidism.

Pharmacology of propylthiouracil in thyrotoxicosis and chronic renal failure

The simultaneous occurrence of thyrotoxicosis and renal failure has rarely been reported in the literature, and data concerning appropriate antithyroid drug management in this circumstance are limited. We studied propylthiouracil pharmacokinetics in one such patient basally and while the patient was receiving hemodialysis. On a day when the patient was not receiving hemodialysis, propylthiouracil serum levels were high, but serum propylthiouracil half-life was not prolonged. During hemodialysis, serum propylthiouracil levels were normal, and the time to peak serum levels was delayed; the disappearance of the drug from the serum was normal after hemodialysis was completed. The amount of propylthiouracil that appeared in the dialysate was approximately 5% of the administered dose. These data suggest that propylthiouracil can be administered in standard dosages to patients with thyrotoxicosis and renal failure.

Pharmacology of propylthiouracil (PTU) in pregnant hyperthyroid women: correlation of maternal PTU concentrations with cord serum thyroid function tests

Previous studies have characterized the pharmacology of propylthiouracil (PTU) in normal and hyperthyroid subjects, but there is little information available regarding PTU pharmacokinetics in pregnant hyperthyroid women. We investigated the serum PTU response to an oral dose of PTU in six hyperthyroid pregnant women both ante- and postpartum. The serum PTU profile during the third trimester of pregnancy was qualitatively similar to that in nonpregnant subjects, but serum PTU concentrations were consistently lower in the late third trimester compared with postpartum values. Cord serum PTU concentrations were consistently higher than simultaneously obtained maternal serum PTU concentrations, suggesting slower PTU clearance in the fetus. There was a significant inverse correlation (r = -0.92; P = 0.026) between the maternal serum PTU area under the curve in the third trimester and the cord serum free T4 index.

Graves' disease with neutropenia and marked splenomegaly: autoimmune neutropenia due to propylthiouracil

A 38-year-old man with Graves' disease taking propylthiouracil (PTU) for 6 years developed neutropenia and marked splenomegaly. After subtotal thyroidectomy with discontinuance of PTU the patient remained asymptomatic for the last two and half years. The serum obtained during the period of neutropenia demonstrated opsonic activity to neutrophils of the patient as well as of normal volunteers. This opsonic antineutrophil activity was located in the IgG fraction of the serum. Furthermore, PTU at the concentration (0.1-1.0 micrograms/ml) attainable in the patient's serum significantly stimulated [3H] thymidine incorporation in the patient's lymphocytes. These findings indicate that the patient developed autoimmune neutropenia by producing opsonic antineutrophil antibodies in association with the PTU therapy.

Peroxidase activity in relation to iodide, 17 beta-oestradiol and thioureylene drug uptake in human polymorphoneutrophils

In polymorphoneutrophils (PMNs) phagocytosis is accompanied by an increase in peroxidase activity. Accumulation of iodide, thioureylene antithyroid drugs and 17 beta-oestradiol also occurs during the process. There is no evidence of an active iodide transport system in the PMNs as pertechnetate is not concentrated and neither ouabain nor perchlorate abolishes iodide accumulation. The uptakes of 125I, [35S]PTU and [3H]-17 beta-oestradiol were compared in phagocytosing PMNs and the effects of various compounds examined. In addition, chemiluminescence generation from luminol by PMNs and by horseradish peroxidase was studied. This indicated that uptake of all three compounds could be associated with activation of the peroxidase system, and inhibition of this enzyme system caused a reduction in their accumulation.

Serum propylthiouracil concentration in patients with Graves' disease with various clinical course

The serum levels of propylthiouracil (PTU) were determined by radioimmunoassay in 10 normal subjects and in 11 patients with Graves' disease after a single 100 or 200 mg oral dose of PTU. The serum half-life of PTU in the normal subjects and in hyperthyroid patients with uneventful clinical course was 75 +/- 19 min (mean +/- SD, n = 6) and 73 +/- 13 min (n = 7), respectively. Maximum serum PTU concentrations were usually attained within 1 h after a single 200 mg oral dose and at 1 h were 5.3 +/- 1.4 micrograms/ml (3.1 +/- 0.82 X 10(-5) M) in normal subjects (n = 6) and 4.8 +/- 2.4 micrograms/ml (2.8 +/- 1.4 X 10(-5) M) in hyperthyroid patients (n = 7). These between-group differences were not significant. Serum PTU concentrations were low in a pregnant hyperthyroid patient with a weak response to PTU treatment. In another patient, who appeared resistant to PTU therapy, the serum PTU level increased as expected at testing, and it was later confirmed that, during treatment, he had not taken the drug as prescribed. In a patient who developed agranulocytosis due to methimazole and subsequently fever due to PTU, the half-life of PTU was prolonged to about 130 min. These findings suggest that monitoring the serum PTU levels in patients with Graves' disease can be of clinical value in patients who do not respond to treatment. Furthermore, it may provide some clues as to the mechanism by which toxic reaction develops.

Propylthiouracil (PTU) pharmacology in the rat. I. Serum and thyroid PTU measurements by radioimmunoassay

We have developed a highly sensitive and specific RIA for propylthiouracil (PTU) which uses 125I-labeled PTU as the radioactive ligand. At a final antibody dilution of 1:10,000, the detection limit for PTU was 100 pg; cross-reactivity with circulating, urinary, and intrathyroid PTU metabolites was negligible. Using this assay, serum and thyroid PTU levels were determined after short term (1 week) and long term (1 month) PTU treatment at doses of 0.0001-0.05%. Serum PTU was a linear function of the PTU dose (r = 0.99; P less than 0.001), whereas thyroid PTU was a linear function of the logarithm of the PTU dose (r = 0.99; P less than 0.001). Serum PTU levels were higher after 1 month of treatment than after administration for 1 week, probably because steady state conditions were not achieved after 1 week. At several doses, thyroid PTU levels were also higher after 1 month of treatment, but the differences were not as striking as those seen in the serum levels. The pharmacokinetic data are consistent with a multicompartmental model for PTU distribution. The logarithmic relationship between thyroid PTU and PTU dose suggests a saturable uptake mechanism for PTU by the thyroid; inhibition of thyroid PTU uptake by PTU itself could also explain these observations.

Effect of alcohol on propylthiouracil disposition

We examined the effect of ethanol on propylthiouracil (PTU) disposition in normal subjects and a rabbit. The drug is metabolized by conjugation. In six normal subjects 19.2 gm oral ethanol, taken either with or two hr after 300 mg PTU, did not change maximum concentration, time to maximum concentration, or total or free AUC of PTU. Pretreatment with ethanol and supplementation to keep blood ethanol concentration above 800 mg/l for 8 hr also did not alter PTU disposition. In the rabbit, the infusion of ethanol (8.81 mg/min) 4 hr after the beginning of an intravenous infusion of PTU 0.05 mg/min did not alter the plasma concentration profile. These results indicate that short-term ethanol dosing does not affect PTU disposition and therefore dosage adjustment is not necessary in patients who drink alcohol.

Correlation between antithyroid effect and serum concentrations of propylthiouracil in patients with hyperthyroidism

1 Correlation between the kinetics of propylthiouracil and its antithyroid effect was studied in 17 hyperthyroid patients. The serum concentration of propylthiouracil 1 h after an oral dose of 400 mg of hyperthyroid patients. The serum concentration of propylthiouracil 1 h after an oral dose of 400 mg of the drug was used as kinetic parameter as this concentration from the previous study was found to correlate significantly (r = 0.84, P less than 0.01) with the area under the serum concentration-time curve. 2 After 3 weeks of treatment with 200 mg propylthiouracil three times daily the serum concentration of propylthiouracil was correlated to the decrease in various thyroid parameters such as total and free indexes of serum thyroxine, triiodothyronine and reverse triiodothyronine. 3 Significant correlations were found between the serum concentration of propylthiouracil and all the measured thyroid variables except reverse triiodothyronine. The highest degree of correlation was obtained between serum propylthiouracil and the percentage decrease in total and free indexes of triiodothyronine (r = 0.63 and 0.70, respectively, P less than 0.01). 4 It is suggested that a serum concentration of propylthiouracil above 4 to 5 micrograms/ml 1 h after an oral dose of 400 mg of the drug will secure a sufficient and rapid antithyroid effect during continuous therapy.

Correlation of plasma propylthiouracil levels with inhibition of thyroid hormone synthesis in the rat

The inhibition of thyroid hormone synthesis was studied in relation to plasma levels of 6-n-propyl-2-thiouracil (PTU). Na 125I (5 muCi) was injected i.p. into adult male Sprague-Dawley rats. After 30 min, graded doses of PTU (0.2 mg/kg. 0.1 mg/kg, and 0.05 mg/kg) were similarly injected. Thyroid hormone synthesis was followed by the accumulation of radioactivity into thyroid glands, which were removed at specified time intervals. PTU levels were measured spectrophotometrically at the time of sacrifice. A-ditionally, PTU (35S) was used to confirm blood levels of PTU and also to follow intrathyroidal PTU levels. Plasma PTU levels in excess of 0.18 microgram/ml completely inhibited thyroid hormone synthesis. Levels between 0.14 and 0.09 microgram/ml had a partial effect, and PTU levels less than 0.09 microgram/ml had no effect on thyroid hormone synthesis.

Propylthiouracil in human milk. Revision of a dogma

Propylthiouracil (PTU) concentrations were measured in blood and milk from nine lactating women after oral administration of 400 mg of PTU. 1 1/2 h after PTU ingestion mean serum-PTU reached 7.7 microgram/ml and the mean concentration of PTU in milk reached only 0.7 microgram/ml. The mean total amount of PTU excreted during 4 h was 99 microgram--i.e., 0.025% of the administered dose. One of the suckling babies was studied for 5 months during which the mother received 200-300 mg of PTU daily: there were no changes in any of the thyroid parameters. PTU is not concentrated in human breast milk and recommended dosages to the mother result in minimal and presumably clinically insignificant doses to the suckling infant.

Effects of thyroid status and choice of USP preparation on the pharmacokinetics of propylthiouracil in the beagle dog

In vitro dissolution profile testing of 12 generic 6-propyl-2-thiouracil (PTU) formulations indicated significant differences in their solubilities in water and in 0.1N hydrochloric acid. Two products, representing the extremes of solubility, were subsequently tested for bioavailability. Bioavailability data from euthyroid, hyperthyroid, and hypothyroid beagle dogs indicated that the rate of PTU absorption varied appreciably between the two generic products studied. These results were consistent with the in vitro dissolution data. Comparisons of the pharmacokinetic parameters obtained for the euthyroid beagle dog to those reported for euthyroid man12 indicated that the beagle dog is an adequate model for PTU disposition in man. These combined results suggest that all generic formulations of PTU are not the same and that the most soluble product may be desirable in situations where rapid absorption is important, such as thyroid storm.

High-pressure liquid chromatographic determination of propylthiouracil in human plasma

A high-pressure liquid chromatographic procedure was developed for the determination of prophylthiouracil in plasma. The method utilizes a reversed-phase C18 chromatographic column and UV detection at 280 nm. The method is sufficiently sensitive for most bioavailability and pharmacokinetic studies. Coinentrations between 0.1 and 5.0 microgram/ml were measured with a coefficient of variation of 0.6-5.3% for a given day. Methylthiouracil was employed as an internal standard.

High-performance liquid chromatographic determination of plasma propylthiouracil

A reversed-phase high-performance liquid chromatographic analysis was developed for propylthiouracil in plasma (1 ml). After protein precipitation with acetonitrile, the solution was diluted with water and injected into a liquid chromatograph equipped with C-18 and C-8 columns in series. The peak area was linear over the 0.25-10-mg/liter range, and the recovery was 101 +/- 4.5%. This assay has the advantages of specificity, simplicity, and speed over previously published methods and requires smaller sample volumes. None of 19 drugs tested interfered with the assay.

Bioavailability of propylthiouracil: Interindividual variation and influence of food intake

The bioavailability of 6-propylthiouracil (PTU) has been examined in eight healthy volunteers, with respect to interindividual variation and influence of food intake. PTU was given as a single oral dose, both in a fasting state and together with a standardized breakfast. Numerous venous blood samples were taken during 5 hours after PTU ingestion, and the concentration of unmetabolized PTU in serum was determined by a specific gas-chromatographic technique. The observations indicate that the amount of PTU absorbed is subject to a large interindividual variation, and that concomitant food intake may exert a minor and non-systematic influence on PTU absorption. Hence, the major issue in PTU therapy is the individualization of the size and interval of dosage. Testing of single-dose PTU kinetics in patients would apparently be helpful.

35-S-antithyroid drug concentration and organic binding of iodine in the human thyroid

35S-methimazole (MMI), 35S-carbimazole or 35S-propylthiouracil (PTU) were given orally to fifty-five patients at various times up to 12 h before surgical thyroidectomy. The amount of 35S radioactivity and labelled drug in thyroid and plasma samples was measured. Intrathyroidal inhibition or organic binding of iodine by MMI, carbimazole and PTU was measured after intravenous administration of 131I, 132I or 125I-iodide. After administration of 35S-carbimazole or 35S-MMI the thyroid to serum (T/S) ratio of 35S radioactivity was greater in thyrotoxic glands than in non-toxic adenoma tissue. 35S-MMI was found in thyroid and plasma samples after administration of 35S-carbimazole. The T/S 35S-MMI was greater than 1 in most but not all patients. 35S radioactivity was also concentrated in the thyroid after administration of 35S-PTU. In thyrotoxic glands there was an 80% inhibition of iodine organification in patients receiving MMI and 60% for those receiving PTU. It is suggested that carbimazole and MMI can be given once or twice daily in some patients but PTU would be less suitable for this dose schedule.

Pharmacodynamics of propylthiouracil in normal and hyperthyroid subjects after a single oral dose

Serum levels of propylthiouracil were measured in 8 normal persons and in 7 patients with hyperthyroidism after a single, 300 mg, oral dose of 6-n-propyl-2-thiouracil (PTU). The patients with hyperthyroidism were restudied after 3, 6, and 9 weeks of individualized treatment with PTU. The serum half-life of the drug in normal subjects was 1.65 h. In patients with hyperthyroidism the serum half-life was similar, and it did not change significantly as the euthyroid state was achieved.

Altered plasma half-lives of antipyrine, propylthiouracil, and methimazole in thyroid dysfunction

In normal, nonmedicated volunteers and in patients with thyroid disorders the plasma half-lives of antipyrine, propylthiouracil, and methimazole were determined after single oral doses. The plasma half-liver plus or minus S.D. of antipyrine, propylthiouracil, and methimazole were 11.9 plus or minus 1.4 hr, 6.7 plus or minus 1.0 hr, and 9.3 plus or minus 1.4 hr, respectively, in normal volunteers, but were shortened to 7.7 plus or minus 1.2 hr, 4.3 plus or minus 0.7 hr, and 6.9 plus or minus 0.6 hr, respectively, in hyperthyroid patients. In hypothyroid patients the plasma half-lives of these drugs were prolonged to 26.4 plus or minus 4.0 hr, 24.7 plus or minus 34.5 hr, and 13.6 plus or minus 4.8 hr, respectively. Return to the euthyroid state restored plasma half-lives to or toward normal. Alterations in plasma drug half-lives during thyroid dysfunction appear to result mainly from accelerated hepatic microsomal drug metabolism in hyperthyroidism and retarded drug biotransformation during hypothyroidism.