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Lower iodine concentrations in the urine was independently associated with higher testosterone levels, a cross-sectional study found.

Looking at nearly 3,000 U.S. men, those with lower urinary iodine concentrations had significantly higher total testosterone levels than men with either normal or high iodine levels (β -0.028, 95% CI -0.049 to -0.006, P=0.01), Arcangelo Barbonetti, PhD, of the University of L'Aquila in Italy, and colleagues detailed in a research letter.

This model was adjusted for a host of factors including several metabolic variables like BMI, blood pressure, glucose, triglycerides, HDL cholesterol, creatinine, plus demographic factors. Low iodine concentrations were considered under 100 μg/L, normal between 100-299 μg/L, and high 300 μg/L or over.

Men falling into the lowest iodine category had median total testosterone levels of 446.70 ng/dL, while those in the normal and high categories had testosterone levels of 398.68 ng/dL and 398.50 ng/dL, respectively. The same pattern was seen when looking at calculated free testosterone, with levels of 75.80 pg/mL, 72.10 pg/mL, and 67.20 pg/mL for the low, normal, and high iodine categories, respectively.

"While an interaction between iodine intake, thyroid hormones, and testosterone cannot be definitively ruled out, excess iodine could affect testicular steroidogenesis directly," the researchers posed. Men in the low iodine category also had the lowest levels of thyroid dysfunction compared with the normal and high categories (8 vs 14 vs 12), though the differences weren't statically significant.

Even though this link falls in line with prior , Barbonetti told 番茄社区app he was still surprised to see these findings, "as they suggest that iodine, which is an element traditionally considered harmless and beneficial, can be potentially harmful to health if in excess."

Barbonetti's group explained that in rats fed a high-iodine diet, iodine accumulated in the testis, which triggered oxidative stress inhibiting activity that led to lower testosterone.

"The results of this study suggest that excess iodine is not harmless and may be associated with extra-thyroid disorders, including androgen deficiency," said Barbonetti. He added that in countries with iodine sufficiency, "the use of medical therapies or diagnostic procedures that bring an iodine load to the patient should be cautious, especially in the presence of comorbidities that may promote the occurrence of male hypogonadism."

"Regulatory agencies should be aware that [iodine sufficiency] is now achieved in most countries, and, hence, in some geographical areas, exposure to excessive levels of iodine can easily be reached," he said. "The mode and extent of [iodine prophylaxis] should probably be reconsidered."

A total of 2,934 men (mean age 47.1) were included in the final analysis. The researchers pulled data from five cycles of the National Health and Nutrition Examination Survey -- 1999 to 2002 and 2011 to 2016 -- which included data on total testosterone and urinary iodine concentration. Testosterone was measured by isotope dilution high-performance liquid chromatography-tandem mass spectrometry and iodine concentration by inductively coupled plasma dynamic reaction cell mass spectroscopy.

Men with lower urinary iodine concentrations tended to be younger and have better metabolic and glycolipid profiles, such as a lower BMI.

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