Prenatal iodine deficiency does not cause enlarged testes

If pregnant women produce too little thyroid hormone, then their sons will have enlarged testes as they mature. In any case, this is reported in medical manuals with great authority. But Dr. Eddy Rijntjes of Wageningen University discovered that this is not true. An incorrect research method has misled researchers for many years.

One-fourth of all children in the world are born with the condition that is similar to that of the rats with which Rijntjes worked. The PhD student at the Human and Animal Physiology Group gave animals feed which contained only a fraction of the quantity of iodine in normal feed. ‘As a result, the concentration of active thyroid hormone in the rats was 60% lower than normal', says the PhD student. ‘If you believe the figures from the WHO, then more than 30% of the world population is deficient in iodine. Every year, especially in developing countries, 250,000 children are born who have a thyroid status which is even worse than this.’ 
Scientists do not know exactly why this is, but it is especially pregnant women who are susceptible for iodine deficiency; as a result they do not produce sufficient quantities of thyroid hormone. In medical terms, they have a hypothyroid condition. 

‘Thyroid hormone is important for the development of the Leydig cells in the testes’, explains Rijntjes. ‘These are the cells that produce testosterone. Because these cells do not fully develop in sons of mothers who have excessively low thyroid hormone levels, the boys produce too little testosterone during their puberty. In that case, they suffer from enlarged testicles. At least that is what we thought for many years.’ 
However, in the animal experiments on which the scientists based their conventional wisdom, the researchers did not use iodine-deficient feed, but the antithyroid drug propylthiouracil. ‘This drug also affects the testosterone-producing Leydig cells in the testis’, says Rijntjes. ‘This is why the researchers thought that the rats who had an iodine deficiency at the beginning of their lives would always suffer from its consequences. But my experiments showed that the effects of prenatal iodine deficiency are not as drastic as previously assumed. If I returned my experimental animals to standard feed before they entered puberty, their testes ultimately developed in a completely normal fashion. 

Rijntjes’ PhD research means that hundreds of thousands of mothers with a low thyroid hormone level have one thing less to worry about. ‘But this does not mean that a thyroid hormone deficiency during pregnancy does not have any health effects’, emphasises Rijntjes. ‘Hypothyroidism also reduces the development of brain cells. And this effect is permanent.’ 

Eddy Rijntjes defended his doctoral thesis on 17 June; his doctoral supervisor was Prof. Wouter Hendriks, Professor of Animal Nutrition. / Willem Koert