The effect of dietary ingestion of oxalic acid on thyroid function in male and female Long-Evans rats

Male and female Long-Evans rats revealed decreased body weights and restricted growth rates when placed on a diet of Purina laboratory chow which was supplemented with 2.5 and 5.0% oxalic acid for a period of 70 days. Male and female rats ingesting 5.0% oxalic acid in the diet showed severe losses in body weight. Significant reduction in thyroid weight, 24-hr thyroid uptake of 125I, and plasma radioactivity were observed together with a significant increase in plasma thyroid-stimulating hormone in male and female rats ingesting dietary 5.0% oxalic acid. Chromatographic analysis of thyroid hydrolysates revealed an increase in thyroid labeling of monoiodotyrosine (MIT) which was associated with a decrease in labeling of diiodotyrosine (DIT) leading to elevation in MIT : DIT ratios. While the labeling of tetraiodothyronine (T4) was unaffected, a significant increase in triiodothyronine (T3) resulted in elevation of the T3 : T4 ratios. These results indicate that chronic ingestion of oxalic acid at the level of 5.0% can lead to a state of hypothyroidism.