Stereospecific determination of D-thyroxin and L-thyroxin in serum

Enantioselective resolution of thyroxine hormone by high-performance liquid chromatography utilizing a highly fluorescent chiral tagging reagent

A highly fluorescent chiral tagging reagent, 4-(3-isothiocyanatopyrrolidin-1-yl)-7-(N,N-dimethylaminosulfonyl-2,1,3-benzoxadiazole, [R(-)-DBD-PyNCS], was employed to develop an indirect resolution method for efficient separation of thyroxine enantiomers,D-T(4) and L-T(4). The reaction of R(-)-DBD-PyNCS with the thyroxine enantiomers proceeds effectively at 40 degrees C for 20 min in the presence of basic medium to produce the corresponding pair of diastereomers. No racemization occurs during the tagging reaction under the optimized conditions. Various experimental parameters for derivatization reaction including the species of catalyst, the concentration of tagging reagent and reaction temperatures, have been examined to get a highest yield for T(4) derivatives. The structure of T(4) derivatives was identified based on ESI-MS/MS measurements in negative mode. The efficient separation of D-, L-T(4) derivatives was achieved by isocratic elution with water-acetonitrile mobile phase containing 1% AcOH on a reversed phase column utilizing a conventional fluorescence detector. The resolution (Rs) value of the diastereomers derived from thyroxine was 5.1. The calibration curves of both the D-T(4) and L-T(4) were linear over the concentration range of 0.1-20 microg/ml. The limits of detection (S/N = 3) for both D-T(4) and L-T(4) were 0.2 ng per injection. The proposed method was applied to the determination of D-T(4) and L-T(4) in pharmaceutical formulations and human serum samples.

Chiral separation of T3 enantiomers using stereoselective antibodies as a selector in micro-HPLC

This work deals with the application of stereoselective antibodies against L-T3 as a tailor-made chiral selector in micro-HPLC. The separations were performed in microbore columns using commercially available anti-L-T3 antibodies chemically bonded to 5 microm silica gel. The enantiomers of T3 were baseline separated under mild continuous isocratic elution conditions using 10 mM phosphate buffer, pH 7.4. The D-enantiomer eluted with the void volume, while the L-enantiomer was retained by the antibody phase and eluted second. An indirect competitive and non-competitive enzyme linked immunosorbent assay (ELISA) was used for testing the stereoselectivity of anti-L-T3 antibodies.

Direct separation and quantitative analysis of thyroxine and triiodothyronine enantiomers in pharmaceuticals by high-performance liquid chromatography

A rapid reversed-phase type HPLC method for the simultaneous separation and analysis of D- and L-thyroxine (D- and L-T(4)) and triiodothyronine (T(3)) was developed using a quinine-derived chiral stationary phase and applied for a quantitative assay of the enantiomeric impurity of the drugs in pharmaceutical formulations of levothyroxine. The influence of operating parameters has been studied for the optimization of the separation and also in order to gain an insight into the retention mechanism. Validation of the method included linearity, precision and accuracy which revealed R.S.D. values of <3.3% for intra-assay precision and percent error ranging from -6 to +2.1% for various defined validation samples, proving satisfactory accuracy. Quantitation was performed over the range of 0.5-500 microg ml(-1) with limits of detection and quantitation lower than 0.1 and 0.5 microg ml(-1), respectively, for both analytes. Further, the determination of 0.1% impurity, of D-T(4) as well as L- and D-T(3) in levothyroxine sodium tablets proved to be feasible.

Determination of L-thyroxine in reference serum preparations as the o-phthalaldehyde-N-acetylcysteine derivative by reversed-phase liquid chromatography with electrochemical detection

A simple procedure for the assay of L-thyroxine in serum preparations with D-thyroxine as internal standard is described. The L-thyroxine is extracted with acetonitrile, fractionated on a reversed-phase silica cartridge and analysed by reversed-phase high-performance liquid chromatography of the o-phthalaldehyde-N-acetyl-L-cysteine derivative. This derivative is not fluorescent, but may be detected with suitable sensitivity and selectivity with an electrochemical detector.

Thyrotoxicosis due to pituitary resistance to thyroid hormones. Successful control with D thyroxine: a study in three patients

Selective pituitary resistance to thyroid hormone (PRTH) is responsible for thyrotoxicosis due to inappropriate secretion of TSH. The TSH suppressive action of D-thyroxine (DT4) has been previously documented in euthyroid and hypothyroid subjects. This prompted us to treat with DT4 three patients with PRTH uncontrolled by anti-thyroid drugs (ATD) alone or supplemented with bromocriptine, and whose follow-up had been complicated by atrial fibrillation in two patients. Because of 100% cross-reactivity between the D and L isomers of T4 and T3 in our RIAs, thyroglobulin (Tg) was used as an index of thyroid secretion. Under ATD, TSH and Tg levels were respectively: 35 mIU/l and 670.5 pmol/l (patient 1), 87 mIU/l and 453 pmol/l (patient 2) and 110 mIU/l and 906 pmol/l (patient 3). When DT4 was added (patient 1, 3 mg daily; patients 2 and 3, 2 mg daily) to the same dose of ATD, plasma TSH and Tg levels fell but were still over the upper limit of normal and thyrotoxicosis persisted as illustrated by a recurrence of atrial fibrillation in one patient. When ATD were withdrawn and DT4 given alone (2 mg daily) all symptoms subsided within 1 month while TSH and Tg levels fell within the normal range. TSH normalization was documented within 1 week in one patient.(ABSTRACT TRUNCATED AT 250 WORDS)

Acute and chronic effects of dextro-thyroxine on pituitary thyroid axis and on reverse triiodothyronine production in euthyroid subjects

Serum levels of thyroxine (T4), triiodothyronine (T3), reverse triiodothyronine (rT3) and TSH were measured in euthyroid subjects after a single dose of 4 mg D-thyroxine (DT4) or of 0.25 mg L-thyroxine (LT4). The same parameters and TSH response to TRH were also evaluated in 7 dyslipidemic patients before and after one month of treatment with 6 mg DT4. T4 levels increased about 165% at h 4 after DT4 and only 47% after LT4; T3 levels remained unchanged until h 10 both after DT4 and after LT4; rT3 levels increased almost 179% after DT4 and only 32% after LT4. TSH levels decreased about 30% after both DT4 and LT4. In the long term study similar variations of the same parameters were observed: basal TSH levels decreased and TSH response to TRH was inhibited in all patients but one; T4 levels increased 62%, T3 levels increased 35%, while rT3 levels increased 545%. Our results show that: both acute and long-term treatment with DT4 suppress TSH secretion; DT4 both in acute and in long-term administration, is preferentially dealogenated in the alaninic ring with production of rDT3, instead of in the phenolic ring with production of DT3. This may contribute to explain its lower metabolic activity.

Stereospecific determination and in vivo monodeiodination of thyroxine enantiomers in euthyroid man

To compare in man the absorption, serum disappearance, and peripheral monodeiodination of the thyroxine enantiomers, we studied six euthyroid subjects who, on separate occasions, orally ingested 3 mg of either dextrothyroxine (DT4) or levothyroxine (LT4). We measured the serum concentrations of total T4 (TT4), total T3, and reverse T3 (rT3) by nonstereospecific radioimmunoassay and we determined serum DT4 and LT4 by stereospecific chromatography. Mean serum TT4 levels from 4 hours were significantly greater after LT4 administration. After DT4 administration, stereospecific analysis of serum revealed two T4 peaks that persisted from 2 to 48 hours. The mean serum LT4 level did not significantly change during the 48 hours after DT4 administration. Increases in serum T3 and rT3 were seen from 2 hours after administration of either enantiomer. From 12 hours the levels of both triiodothyronines after LT4 were significantly higher than after DT4. In this short term study we found no evidence that in man DT4 is converted to LT4, nor is it preferentially deiodinated to rT3. The greater and more persistent increases in serum T4 and T3 observed after LT4 probably contribute to the known higher bioactivity of that enantiomer.

Effect of D-thyroxine on serum sex hormone binding globulin (SHBG), testosterone, and pituitary-thyroid function in euthyroid subjects

Concentrations of serum sex hormone binding globulin (SHBG) and free testosterone (T) were examined in 10 euthyroid subjects (5 men and 5 women) before, during and after 30 days of the daily ingestion of 1 or 4 mg D-thyroxine (D-T4), the thyroxine analog that has only 1-15% of the calorigenic effect of L-thyroxine (L-T4). No changes in serum L-T4 or triiodothyronine (T3), serum cholesterol, SHBG, T, progesterone, estradiol-17 beta, or free T concentrations were observed in response to the 1 mg dose, but there was a slight elevation in the free thyroxine index (FTI) and a significant (p less than 0.02) suppression of the thyrotropin (TSH) response to thyrotropin releasing hormone (TRH). The 4 mg dose of D-T4 induced an increase in SHBG levels in all but one man. There was a significant negative correlation between the SHBG and percent free T (p less than 0.05) although the mass of free T did not change. As a group, the women responded with a greater increase in SHBG and decrease in percent free T than the men. Serum cholesterol decreased (p less than 0.01), all serum thyroid hormone values measured by RIA were increased (p less than 0.01), and the TSH response to TRH was completely suppressed. Despite these changes, the subjects remained clinically euthyroid. Concentrations of testosterone, progesterone and estradiol-17 beta remained unchanged. Serum luteinizing hormone (LH), which was evaluated in the men only, also did not change during the daily administration of 4 mg D-T4.