3,3'-Diiodothyronine sulfate excretion in maternal urine reflects fetal thyroid function in sheep.

We have shown that there is significant fetal-to-maternal transfer of sulfated metabolites of thyroid hormone after fetal infusion of a pharmacologic amount of 3,3',5-triiodothyronine (T(3)) or sulfated T(3) in late pregnancy in sheep (Am J Physiol 277:E915, 1999). The transferred iodothyronine sulfoconjugate, i.e. 3,3'-diiodothyronine sulfate (T(2)S), of fetal origin appears in maternal sheep urine. The present study was carried out to assess the contribution of T(2)S of fetal origin to the urinary pool in ewes. Eighteen date-bred ewes (mean gestational age of 115 d) and their twin fetuses were divided into four groups. In group I (control, n = 5), both ewes (M) and their fetuses (F) were sham operated for thyroidectomy (Tx). In group II, the ewes (MTx, n = 4) and, in group III, the fetuses (FTx, n = 4) were subjected to Tx. In group IV (MTx.FTx, n = 5), both the ewe and fetus had Tx. After 10-12 d, fetal and/or maternal hypothyroidism were confirmed by serum thyroxine (<15 nmol/L) measurements. In addition, we infused radioactive T(3) without disturbing the T(3) pool in three singleton near-term fetuses and assessed the amount of radioactive iodothyronine that appeared in maternal urine (MU). After infusing [(125)I-3'],3,5-T(3) via fetal vein to the near-term normal fetuses, radioactive T(2)S was identified as the major metabolite in MU by HPLC and T(2)S-specific antibody. MU T(2)S excretion (pmol/mmol creatinine) was significantly reduced by FTx and MTx.FTx but not by MTx. In addition, positive correlations (p < 0.01) were found between MU T(2)S excretion and fetal serum thyroxine and T(3) concentrations but not with maternal serum thyroxine or T(3) levels. T(2)S of fetal origin contributes significantly to the MU pool.

Iodothyronine sulfotransferase activity in rat uterus during gestation.

In developing mammals, we and others demonstrated that sulfation is an important pathway in the metabolism of thyroid hormone, and there is significant fetal-maternal transfer of sulfated iodothyronine. In the present study, we characterized a novel iodothyronine sulfotransferase (IST) in pregnant rat uterus. (125)I-labeled 3,3'-diiodothyronine (T(2)), T(3), rT(3), and T(4) were used as substrates with unlabeled 3'-phosphoadenosine-5'-phosphosulfate (PAPS) as the sulfate donor. Sulfated iodothyronine products were separated by Sephadex LH-20 column and further identified on reverse phase HPLC. We measured IST activity in pregnant rat uterus by incubating 1 microM substrate, 50 microM PAPS, and 50 microg cytosol protein, pH 7.2, 30 min at 37 degrees C. The results show that the substrate preference of the uterine IST activity is: T(2 )> rT(3 )> T(3)> T(4); the pH optimum is 6.0 for T(2). The K(m) and V:(max) (for gestational day 21 uterus) for T(2) are 0.62 microM and 3466 pmol/mg protein/h, respectively; for PAPS the values are 2.6 microM and 1523 pmol/mg protein/h, respectively. During pregnancy, the total activities exhibit a U-shaped curve with minimum activity at day 13 of gestation; while a thermostable activity increases significantly near term. In summary, there is significant uterine IST that varies during pregnancy. The role of this uterine sulfotransferase activities in regulating the bioavailability of thyroid hormone in the developing fetus remains to be elucidated.

Identification of thyroxine-sulfate (T4S) in human serum and amniotic fluid by a novel T4S radioimmunoassay.

Recently, we identified significant amounts of thyroxine sulfate (T4S) in fetal sheep serum, meconium, bile, and amniotic and allantoic fluids. Little is known, however, about sulfate conjugation of thyroxine in humans. In this study, we employed a novel, sensitive T4S RIA to address this question. The rabbit antiserum was quite specific; T4, T3, rT3, and 3,3'-T2 showed less than 0.002% cross-reactivity. Other analogs cross-reacted less than 0.0001%. Only rT3S and T3S cross-reacted significantly (9.9% and 2.0%, respectively). The mean serum T4S concentration (ng/dL) was 8.6 in euthyroid subjects, 14.4 in hyperthyroid subjects, 5.0 in hypothyroid subjects, 5.9 in pregnancy, and 4.5 in patients with nonthyroid illnesses. T4S concentration in amniotic fluid from women at 18-19 weeks of gestation (25.5 ng/dL) was higher than that at 14-15 weeks of gestation (14.3 ng/dL). A significant rise in serum T4S was detected in hyperthyroid patients 1 day after ingestion of 1 g of ipodate. These data suggest that T4S is a normal component of human serum and amniotic fluid, and it is mostly derived from T4 peripherally and accumulates when type I 5'-monodeiodinating activity is low in fetuses or inhibited by drugs, such as ipodate.

Characterization of thyrotropin-induced increase in iodothyronine monodeiodinating activity in mice.

To further characterize the effect of TSH administration on thyroid iodothyronine monodeiodinating activity, we have evaluated the in vitro conversion of T4 to T3 (outer ring deiodination) and T3 to 3,3'-diiodothyronine (T2; inner ring deiodination) by mouse thyroid, liver, and kidney homogenates, comparing tissues from TSH-treated mice (0.1-200 mU bovine TSH, ip, for 1-3 days) with tissues from saline-treated controls. The in vitro conversion activity was studied in the presence of 1-20 mM dithiothreitol; most of the studies were carried out at 4 mM. Studies were carried out at optimal pH 6.5 for outer ring and 7.8 for inner ring deiodination. The iodothyronine monodeiodinase in mouse thyroid is similar to the ones in liver and kidney. It is heat labile (inactivated at 56 C for 5 min), inhibited by propylthiouracil (0.2 mM) and ipodate (0.2 mM), and unaffected by methimazole (up to 20 mM), ascorbate (up to 0.1 M) or KI (up to 20 mM). The mean +/- SE baseline rates of T4 to T3 and T3 to T2 conversion were 100 +/- 6.3 and 56.5 +/- 2.9 pmol/mg thyroid protein X 30 min at 37 C, respectively. A significant increase in each conversion activity was found after TSH treatment (0.2 U, ip, daily for 3 days); T4 to T3 conversion rose to 282 +/- 15.4, and T3 to T2 increased to 153 +/- 7.4 pmol/mg thyroid protein (P less than 0.001). A 12.8% increase in thyroid weight was found in the TSH-treated group (P less than 0.03 compared with saline control group). Similar but less marked increased in monodeiodinating activities were seen in the liver. A minimal but significant increase in inner ring monodeiodination with no significant increase in T4 to T3 converting activity was found in kidney, which, in the mouse, has markedly less outer ring deiodinase than liver or thyroid. The iodothyronine monodeiodinating activities did not increase until 12 h in thyroid and 48 h in liver after the first dose of TSH. Significant increases in T4 to T3 and T3 to T2 conversion were seen with doses of TSH as low as 0.1 mU (ip, daily for 3 days), and there was a linear dose-response thereafter. The decay of the increased iodothyronine monodeiodinating activities after a single dose of TSH (0.2 U) appeared to be linear, with a decay t 1/2 of 1.3 days for T4 to T3 conversion and about 1.0 day for T3 to T2 conversion.(ABSTRACT TRUNCATED AT 400 WORDS)