Conclusion The thyroid hormone network has a considerable capacity to compensate for disturbances of this feedback system as it is necessary for various metabolic and catabolic reactions and development. With respect to still insufficient nutritive iodine-supply of almost one third of the world population, possible adverse flavonoid actions on the thyroid hormone axis have to be examined more closely.

Thyroid hormone (TH) homeostasis depends on peripheral activation and inactivation of iododthyronines by selenoenzymes of the deiodinase (Dio) family. We genetically inactivated hepatic selenoenzyme expression, including Dio1, in order to determine the contribution of hepatic Dio to circulating TH levels. Serum levels of TSH, total T4, and total T3 were not different from controls. We measured Dio1 and Dio2 in kidney, skeletal muscle, heart, brown adipose tissue, and brain, but did not find compensatory up-regulation in these tissues. Finally, we determined expression in the liver of the following T3 target genes: Spot14, α-glycerophosphate dehydrogenase (αGPD), and malic enzyme (ME). On the transcript level, both Spot14 and αGPD were reduced in Dio deficient liver to about 60–70% of controls. However, mRNA and activity of ME were significantly increased in the same mice. Further studies are directed to elucidate changes in lipid or carbohydrate metabolism in these mice. Together, our results indicate that hepatic Dio1 activity is not absolutely required to sustain the euthyroid state in mice. Our findings are compatible with results from Dio1-KO mice, however, T4 levels are increased in Dio1-KO mice possibly indicating a role of Dio1 in the pituitary.

Endocrine disrupting compounds (EDC) of natural or synthetic origin were originally found to impair development and endocrine regulation of the reproductive system. Here, we assessed whether they also exert effects on the thyroid hormone axis. Female ovariectomized rats were treated for 12 weeks with two substances used as UV absorbers in sunscreens: Octyl-methoxycinnamate (OMC; 66 and 310mg/day) and 4-methylbenzylidene-camphor (4-MBC; 0.4 and 1.74mg/day) alone or in combination with soy-containing food. These substances are known to be resorbed by the skin, circulating in the blood (Janjua et al. 2004J Invest Dermatol 123:57–61) and suspected to have endocrine disrupting effects. The soy isoflavone genistein has estrogenic and anti-estrogenic effects and is known to inhibit thyroid hormone biosynthesis. As type I 5'-deiodinase (5'DI) is a well known T3-regulated endpoint, 5'DI activity was measured by deiodination of 125I--reverse T3, and 5'DI gene expression was determined by real time RT-PCR. Treatment with OMC caused a significant reduction of 5'DI activity from a basal level of 6.6 to 3.0 and 3.1 pmol iodide released x mg–1 x min–1 in the liver in the absence and presence of soy in the diet, respectively, while 4MBC did not show any effects. A similar decrease was also observed for 5'DI mRNA levels. In the kidney OMC as well as 4MBC led to a marked decline of 5'DI activity from 8.4 to 3.0 pmol iodide released x mg–1 x min–1, independent of the diet. Additionally, higher 5'DI activities were found in the kidney of animals consuming soy containing diet, while no such effect was obtained in liver. This data indicates that 4MBC and OMC affect thyroid hormone metabolism via tissue-specific inhibition of 5'DI activity and gene expression. Thus, 4MBC and OMC do not only interfere with steroid-hormone dependent regulation, but may also disturb endocrine regulation via the thyroid hormone axis.

Thyroid hormone (TH) homeostasis depends on peripheral activation and inactivation of iodothyronines by selenoenzymes of the deiodinase (Dio) family. We genetically inactivated hepatic selenoenzyme expression, including Dio1, in order to determine the contribution of hepatic Dio to circulating TH levels. Serum levels of TSH, total T(4), and total T(3) were not different from controls. We measured Dio1 and Dio2 in kidney, skeletal muscle, heart, brown adipose tissue, and brain, but did not find compensatory up-regulation in these tissues. Finally, we determined expression in the liver of the following T(3) target genes: Spot14, alpha-glycerophosphate dehydrogenase (alphaGPD), and malic enzyme (ME). On the transcript level, both Spot14 and alphaGPD were reduced in Dio-deficient liver to about 60-70% of controls. However, mRNA and activity of ME were significantly increased in the same mice. Together, our results indicate that hepatic Dio1 activity is not absolutely required to sustain the euthyroid state in mice.

A variety of (bio-)chemicals originating from synthetic or natural sources influence the endocrine system directly or via secondary routes and have been termed „endocrine disrupters“ (ED) based on original observations on their adverse effects on the reproductive axis. In an attempt to analyse if similar effects are elicited also on the thyroid hormone axis ovarectomized rats were treated with the ED octyl-methoxycinnamate (OMC,12.5g/kg), 4-methylbenzylidene-camphor (4-MBC,12.5g/kg), 4-nonylphenol (4-NPH, 20 or 80mg/kg) for 12 weeks in comparison to estradiol benzoate (E2, 342mg/kg) or 5α-androstane-3β,17β-diol (Adiol,150mg/kg). Additionally the animals were subdivided into groups receiving soy-free and soy-containing food. Hepatic expression of malic enzyme (ME) (activity, Northern and Real-time PCR) was chosen as an endpoint of ED action since ME is known to be regulated by both steroid and thyroid hormones. T4 and T3 serum levels were higher in the soy-free group and were further increased by E2 and 4-NPH. MBC decreased T4 and elevated T3 levels suggesting stimulation of 5’-deiodinase activity. ME activity was stimulated in rats treated with E2 (p<0.001), E2+soy (p<0.001), Adiol (p<0.01) and OMC+soy (p<0.01) whereas in soy-treated animals a slight reduction (p<0.05) was observed. No significant alteration in ME activity was found for the other treatments. In parallel the ME RNA levels of E2 and Adiol treated rats were elevated (E2: 2-fold, p<0.05; Adiol: 2-fold, p<0.05) and decreased in the 4-MBC+soy group (0.5-fold, p<0.001). Soy and 4-NPH+soy had no effect on ME RNA levels. Thus T3, E2 and Adiol influence ME expression on the transcript level. Since 4-MBC+soy decreases ME RNA levels without affecting ME activity it might be possible that certain EDs repress ME-gene-transcription while others stimulate ME activity posttranscriptionally.

To assess interference with endocrine regulation of the thyroid axis, rats (female, ovariectomised) were treated for 12 weeks with the suspected endocrine active compounds (EAC) or endocrine disrupters (ED) 4-nonylphenol (NP), octyl-methoxycinnamate (OMC) and 4-methylbenzylidene-camphor (4-MBC) as well as 17β-estradiol (E2) and 5α-androstane-3β,17β-diol (Adiol) on the background of a soy-free or soy-containing diet, and endpoints relevant for regulation via the thyroid axis were measured. Thyrotropin (TSH) and thyroid hormone (T4, T3) serum levels were altered, but not in a way consistent with known mechanisms of feedback regulation of the thyroid axis. In the liver, malic enzyme (ME) activity was significantly increased by E2 and Adiol, slightly by OMC and MBC and decreased by soy, whereas type I 5′-deiodinase (5′DI) was decreased by all treatments. This may be due rather to the estrogenic effect of the ED, as there is no obvious correlation with T4 or T3 serum levels. None of the substances inhibited thyroid peroxidase (TPO) in vitro, except for NP. In general, several endocrine active compounds disrupt the endocrine feedback regulation of the thyroid axis. However, there was no uniform, obvious pattern in the effects of those ED tested, but each compound elicited its own spectrum of alterations, arguing for multiple targets of interference with the complex network of thyroid hormone action and metabolism.