Effects of triiodothyronine on turnover rate and metabolizing enzymes for thyroxine in thyroidectomized rats.

AIM: Previous studies in rats have indicated that surgical thyroidectomy represses turnover of serum thyroxine (T4). However, the mechanism of this process has not been identified. To clarify the mechanism, we studied adaptive variation of metabolic enzymes involved in T4 turnover. MAIN METHODS: We compared serum T4 turnover rates in thyroidectomized (Tx) rats with or without infusion of active thyroid hormone, triiodothyronine (T3). Furthermore, the levels of mRNA expression and activity of the metabolizing enzymes, deiodinase type 1 (D1), type 2 (D2), uridine diphosphate-glucuronosyltransferase (UGT), and sulfotransferase were also compared in several tissues with or without T3 infusion. KEY FINDINGS: After the T3 infusion, the turnover rate of serum T4 in Tx rats returned to normal. Although mRNA expression and activity of D1 decreased significantly in both liver and kidneys without T3 infusion, D2 expression and activity increased markedly in the brain, brown adipose tissue, and skeletal muscle. Surprisingly, hepatic UGT mRNA expression and activity in Tx rats increased significantly in comparison with normal rats, and returned to normal after T3 infusion. SIGNIFICANCE: This study suggests that repression of the disappearance of serum T4 in rats after Tx is a homeostatic response to decreased serum T3 concentrations. Additionally, T4 glucuronide is a storage form of T4, but may also have biological significance. These results suggest strongly that repression of deiodination of T4 by D1 in the liver and kidneys plays a major role in thyroid hormone homeostasis in Tx rats, and that hepatic UGT also plays a key role in this mechanism.

Influence of iodine-deficiency on thyroid hormones homeostasis in rats.

Little is known about the kinetics and metabolism of thyroid hormones in the hypothyroid state. In order to optimize hormone replacement therapy, it is important to understand variations in the kinetics and metabolism of thyroid hormones. To investigate these factors, we monitored serum thyroxine (T4) and triiodothyronine (T3) levels in iodine-deficient diet (ID) rats using online solid-phase extraction liquid chromatography-mass spectrometry/mass spectrometry (Online SPE LC-MS/MS). Furthermore, we evaluated supply and turnover rates of T4 in ID rats using a stable isotope-labeled T4 ([¹³C9]T4). Although serum T4 levels gradually declined after beginning ID treatment, T3 levels were unchanged throughout the experimental period. After intravenous administration of [¹³C9]T4 to ID rats, [¹³C9]T4 levels were monitored. We previously reported that significant differences of supply and turnover rates for T4 were observed in surgically thyroidectomized (Tx) rats. Surprisingly, there were no differences of supply and turnover rates for T4 between ID rats and intact rats. In conclusion, there were significant differences of supply and turnover rates for T4 between the hypothyroid states of ID and Tx rats. In ID rats, T3 might be preferentially biosynthesized in the thyroid, and ID treatment might not affect T4 kinetics. Our method, online SPE LC-MS/MS monitoring using a stable isotope tracer, has the potential to be used as a diagnostic tool to investigate the pathogenesis of thyroid disease and is valuable for optimizing the dosage in thyroid hormone replacement therapy.

Influence of thyroidectomy on thyroxine metabolism and turnover rate in rats.

Little is known about the kinetics and metabolism of thyroid hormones in the hypothyroid state. To investigate these factors, we developed a reliable method for measurement of serum thyroxine (T(4)), triiodothyronine (T(3)), reverse-T(3) (rT(3)) and stable isotope-labeled T(4) ([(13)C(9)]T(4)), using online solid-phase extraction liquid chromatography-mass spectrometry/mass spectrometry (online SPE LC-MS/MS). We measured supply and turnover rates of T(4) in thyroidectomized (Tx) rats using [(13)C(9)]T(4) as a tracer. In rats, serum T(4), T(3) and rT(3) were decreased but not completely ablated after surgical Tx. Endogenous T(4) and T(3) levels in Tx rats were maintained at a constant low level throughout the experimental period. [(13)C(9)]T(4) levels declined with a half-life of ∼1.2 days after it was administered to Tx rats intravenously. These findings strongly suggest that serum T(4) levels in Tx rats are maintained by T(4) supplied by extra-thyroidal tissues (e.g. secretion of extra-thyroidal storage, enhancement of enterohepatic recirculation, and production in extra-thyroidal tissues). Moreover, the turnover rate of T(4) in Tx rats was approximately twofold lower than in controls. This finding suggests that degradation of serum T(4) is repressed by Tx. In conclusion, serum T(4) is maintained at a constant low level by T(4) supply from extra-thyroidal tissues and repression of T(4) degradation in Tx rats. The powerful online SPE LC-MS/MS tool can be used to investigate thyroid hormones kinetics and metabolism, and thus has the potential to be used as a diagnostic tool and to investigate the pathogenesis of thyroid disease.