An increase in the number of people at the population level who have symptoms of apathy  and low sex drive  could be due to an increase the number of people who have a thyroid problem whether the problem is due to iodine deficiency or from other causes. There is more research available about iodine deficiency as a cause of hypothyroidism but there has also been research suggesting that an excess of goitrogens may also cause epidemic levels of thyroid problems. Cassava is an example of a goitrogenic food that has affected large numbers of people in areas where it is frequently consumed. Iodine supplementation can help against the effects of many goitrogens which can “include sulfur-bearing organic compounds, industrial pollutants, and artificial and bacterial contaminants of water.” And protein-energy malnutrition and vitamin A deficiency may affect iodine status. 
Radioactive iodine may also affect thyroid health. And exposure to radioactive iodine may be affecting more people than is recognized. The mind is connected to the body after all, so looking for physical reasons for mental health symptoms makes more sense to me than assuming that apathy and sexless relationships have simply become the new normal for a significant percentage of people. Fatalism is a term being used to describe the attitude being exhibited by many Japanese young adults.  But the term fatalism might also just be a new way to describe apathy.
In a report released in 2013, the Fukushima Prefecture Health Management Survey found that more than 40 percent of the Japanese children living in the area showed evidence
of thyroid nodules or cysts. None of the children were found to have thyroid cancer. Thyroid hormone status was not mentioned.  The presence of nodules or cysts is not uncommon according to other studies, and have been found at a frequency of 67% in some research; the importance of assessing for iodine deficiency was mentioned in the abstract. 
Before Fukushima there was Hiroshima and Nagasaki.  It’s sad to consider the possibility of large numbers of people being affected by a chronic health condition but I also find the idea sad that fatalism might be affecting large numbers of people. Having a fatalistic mood seems far less treatable than having an actual physical condition. Thyroid hormone replacement might be possible if hypothyroidism is diagnosed. Infants who are born with congenital hypothyroidism can develop normally if the condition is diagnosed early and the infant receives adequate thyroid hormone or iodine depending on the reason for the hypothyroidism. If infants with congenital hypothyroidism are left undiagnosed and untreated then they are more at risk to grow to a shorter height and have a lower IQ than typical.
Iodine supplements can help protect against radioactive iodine if taken before damage to the thyroid has occurred but too much iodine may be harmful if hyperthyroidism is a risk. See a health professional if concerned about iodine or thyroid status. Lab tests are available to assess iodine and thyroid hormone levels and whether there are thyroid autoimmune antibodies present.
Being able to see a medical doctor about a physical condition might get more effective results than seeing a psychiatrist about fatalism if the apathetic mood is actually a mental health symptom that is being caused by a physical condition.
/Disclaimer This information is provided for educational purposes and is not intended to provide individual health care. Please see a health professional for individualized health care./