The effects of taking chronic nitrate by drinking water on thyroid functions and morphology

Abstract

Nitrate incorporation in humans takes place via drinking water and food. The water used for drinking and cooking in the goitrous areas is high in nitrate content. The aim of the present study was to evaluate both chronic effects and the dose-response relationship of nitrate on thyroid functions. A total of rats were divided into 5 work groups and sodium nitrate was added to their drinking water in different concentrations (0, 50, 100, 250 and 500 mg/l) over a 30-week period. The radioiodine uptake of thyroid was decreased in the 50 mg/l nitrate group, whereas it was increased in the 250 and 500 mg/l nitrate groups as compared to control. All hormones of thyroid gland except total thyroxine and thyroid-stimulating hormone were decreased in the 50, 250 and 500 mg/l nitrate groups. However, the level of total thyroxin was increased in the 100 mg/l nitrate group. Thyroid gland weights were increased in all experimental groups. Histomorphological changes were observed in the 250 and 500 mg/l nitrate groups. These findings suggest that nitrate impairs thyroid function involving the hypothalamo-hypophysio-thyroid axis. This observation could contribute to the current discussion about the acceptable daily intake of nitrate, as well as drinking water nitrate standard safety margins.

Effects of elevated nitrate on endocrine function in Atlantic salmon, Salmo salar

“…In zebrafish (Danio rerio), exposure to perchlorate, another NIS inhibitor, caused clear changes in thyroid histology, but no significant reduction in T 4 concentration (Mukhi et al, 2005). In contrast, female Wistar rats exposed to various concentrations of NO 3 − in their drinking water for 30 weeks showed depressed TH concentrations at exposures as low as 50 mg/L NO 3 − (Eskiocak et al, 2005). It is possible that a 27-day exposure to elevated NO 3 -N was not sufficient to elicit a response in TT 4 or TT 3 in the current study.…”

“…Nitrate is a goitrogen and interferes with the production of thyroid hormones (THs) by competitively binding to the sodium/iodide symporter (NIS) on thyroid follicles, decreasing the availability of iodide (I − ) for use in TH synthesis (De Groef et al, 2006;Lahti et al, 1985;Tonacchera et al, 2004). Female Wistar rats exposed to NO 3 − in their drinking water for 30 weeks experienced significant reductions in total triiodothyronine (TT 3 ) at doses of 50, 250 and 500 mg/L NO 3 − (Eskiocak et al, 2005). In fishes, exposure to elevated NO 3 − resulted in significantly lower iodine-125 ( 125 I) uptake by a number of tissues including thyroid (Lahti et al, 1985).…”

Chronic exposure to nitrate significantly reduces growth and affects the health status of juvenile Nile tilapia (Oreochromis niloticus L.) in recirculating aquaculture systems

“…In mosquitofish, embryonal dry weight was reduced and reproductive behaviour of mature females was affected at minimal concentrations of 5 mg L −1 ‐N (Edwards, Miller & Guillette ). Moreover, elevated nitrate concentrations up to 110 mg L −1 ‐N lead to a decrease in the thyroid hormones T3 and T4 in rats (Eskiocak, Dundar, Basoglu & Altaner ). The impact on swimming performance and survival in juvenile rainbow trout has already been reported at 91 mg L −1 ‐N (Davidson et al .…”

Exposición a nitratos en agua y su relación con disfunción de la glándula tiroides: revisión sistemática ¿Existen riesgos para la salud de la población?

“…Así, en los modelos animales, se han observado cambios morfológicos importantes que explican el bocio, dentro de los cuales destacan: vacuolización y un aumento del tamaño de los folículos con un aplanamiento del epitelio folicular tiroideo ante exposiciones a concentraciones superiores a 150 mg/l 7 . Al comparar marcadores de disfunción tiroidea entre los grupos de ratas expuestos y no expuestos, estos estudios demuestran mayor peso de la tiroides en todos los grupos expuestos, independiente de la concentración de nitrato administrada y disminución de la actividad de la peroxidasa tiroidea (enzima que cataliza la reacción del ion yoduro a yodo para su incorporación a las precursoras de las hormonas tiroideas en la glándula), disminución de T4 total, sin resultados consistentes respecto a T3, observándose concomitantemente un aumento de la TSH y excreción urinaria de yodo, dando cuenta de la menor capacidad de la tiroides de utilizar este ion para sintetizar las hormonas 1,7,8 . Respecto a estudios en humanos, destaca un sólo estudio clínico randomizado, realizado en los Países Bajos en el año 2007, en el cual se dividen los individuos en 2 grupos de 10 personas, a los cuales se expone a nitrato en altas concentraciones (15 mg/kg por 28 días a beber en agua) junto a una dieta restringida en yodo, mientras que, al otro grupo se le entrega un placebo (agua destilada), siguiendo la misma dieta que el grupo experimental.…”