We found that from 1992 to 2017, the incidence of ASIR of TC in China and in the U. S. was higher in women than in men and still at a relatively higher level. We observed the incidence of TC varies by gender, and estrogen levels of women may be one of the risk factors for TC . Join-point regression analysis presented incidence rates increased in both men and women from 1992 to 2017, while incidence rates obviously increased in Chinese men mainly from 2006 to 2011. Due to the interaction between age, period and cohort effect, we use an age-period-cohort regression model and internal estimator algorithm to analyze and estimate the change of TC. In addition, the incidence of TC in the model is significantly related to the age effects and cohort effects. Therefore, it is necessary to further analyze the causes and differences of these trend changes in the model, and further discuss the risk factors leading to the occurrence of TC disease. In general, the incidence of TC increases with age and period, especially the 40–75 age groups of men in the U.S. The men showed a significant upward trend from 2007 to 2017, but decreased with the birth cohort in China.
Based on our findings, the incidence of TC increased with the age of men and women, and China's aging may exacerbate this situation . Chinese men have a higher risk of TC in the 40–55 age group, probably because residents between the ages of 40–59 paid more attention to physical examination, and the detection rate in cities are higher than in rural areas . The rapid rise in age RRs of Chinese men for the 70–79 age group is due to the most variability of cancer prognosis in the old ages and the highest risk of TC treatment. Among the female population in China, the fastest growth was in women of childbearing age, which is consistent with the results of other countries . The rapid increase RRs of 20–30 age groups of women may be an annual obstetric and gynecological examination during reproduction in China. The risk of TC in women increases during puberty, but decreases after menopause. Among young women, the incidence of TC ranks first among malignant tumors. It is suggested that hormone factors are involved in TC, and estrogen increases thyroid growth . According to our results, old age people had a higher risk of TC than young people did. The age effects may increase the risk of illness in the elder people. In addition, increased complications of TC are existed in older TC patients than in younger patients. China’s aging is growing faster than the U.S., the trend of population growth and aging is increasing in China . Therefore, we may pay more attention to prevention and control the occurrence of TC in the old age people.
The RRs of TC in women increased rapidly with the age of before 45-year-old age group in the U.S. However, the RRs of TC decreased rapidly after the 60-year-old, which may indicate that estrogen levels play a role in the development of TC. There was obvious gender differences in the incidence of TC. Women were 2–3 times more likely than men for TC incidence from 1992 to 2017. It further indicates that TC may be related to estrogen . The age RRs of TC for U.S. men increased slowly from 20 to 49 years but increased rapidly after 50 years of age. Therefore, among men over 50 years old, the diagnosis rate of papillary TC is high, but over diagnosis is rare among men between 20 and 49 years old in the U. S. Among patients with TC in the U. S., the majority of patients in diagnosis were 45–49 age groups for women and 55–59 age groups for men . The difference in age and gender may be that middle-aged women use more health care services than men. This difference is also a result of reproductive activity and menopause, leading to earlier diagnosis [7, 27]. Moreover, compared with women, men tend to pay more attention to their health when they get old. According to a study, the increased TC incidence rates have been reported among young people and adults in the U.S. , which shows that some risk factors, such as high body mass index (BMI),  ionizing radiation,  may have contributed to increasing TC incidence rate.
Period effects are usually affected by a complex set of historical events and environmental factors. Environmental factors are closely related to TC, that is, exposure to ionizing radiation is one of the TC risk factors [31, 32]. On the whole, the period RRs of men and women in China is on the rise. From 1992 to 2008, the frequency of medical diagnostic and therapeutic nuclear medicine in China has been increasing, resulting in a significant increase in the annual personal radiation dose . It is not difficult to find that the period effect of men and women after 2007 in China has a clear rising trend. Among them, men showed a slow upward trend after 2012, while women increased significantly from 2007 to 2017. A previous study conducted in China reported that in 2008, about 17% of people over the age of 15 had regular physical examinations, 68% had a physical examination every 7–12 months, and one-third of the population conducted X-rays . More frequent nuclear medical examinations may lead to increasing radiation exposure, resulting in an increased incidence of TC. With the acceleration of China's industrialization process, the exposure to risk factors is becoming more and more obvious . From 1992 to 2017, men’s carcinogens increased significantly. The rising incidence of men may be due to cancer caused by occupational diseases . Over the past 25 years, environmental exposure caused by industrial activities of industrial density may have affected the incidence of TC, which is an occupational carcinogen. TC is considered a cancer that can be affected by occupational exposure. In addition, the GBD study also showed that cancer death and disability-adjusted life years (DALYs) due to occupational risks began to increase around 2007. In fact, occupational exposure has increased since 2007 . Especially, male workers who are exposed to certain solvents and pesticides have an increased risk of developing thyroid tumors. Period RRs showed a remarkable increasing trend of Chinese men, which may be caused by occupational carcinogens. Therefore, it is necessary to minimize the exposure of men to occupational carcinogens in China .
In the U. S., men and women period effects have a similar upward trends. TC is one of the ten most common cancers in the U.S. From 1990 to 2017, the incidence of TC has been increasing at a higher rate than any other malignant tumor. The survey data of the U.S. from 2003 to 2009,revealed that the 5-year survival rate of TC reached up to 98.2% . The steady growth trend of men and women may be due to the rise in the economy of the U.S. The increased incidence of TC was caused by over diagnosis from 1990 to 2000 . The socio-economic development of the U.S. is rapidly increased. In general, people with higher socioeconomic status are more likely to access health care services, and have increased contact with the health care system, leading to an increased risk of over diagnosis. A positive correlation is existed between socioeconomic status and TC risk .
The cohort effect reflects the influencing factors that appear early in life. Exposure to certain risk factors early in life may adversely affect future life . The birth cohort effect obtained through the APC model is the net effect after excluding the age effect and the period effect. Men and women simultaneously increased the risk of morbidity in the cohort effect. This may be linked to China's historical background and economic development. One stage was from 1943–1947 to 1948–1952, which was considered to be related to the Japanese invasion over China and plunged China into a long war. People's living environment has deteriorated, medical and health services have been severely damaged, and health cannot be guaranteed. Moreover, during the periods of 1958–1962 and 1963–1967, China experienced a series of social and economic system changes, plus the three years of famine from 1959 to 1961, the food shortage was very serious and health risk factors have increased significantly . In terms of gender differences in China, we found that in the early cohort women had a lower risk of incidence than men. After the birth cohort in 1933–1937, the risk of women in the incidence began to be higher than that of men, and in the birth cohort after 1978–1982, the risk of women began to be lower than that of men. It is clear that women greatly reduced the risk of the birth cohort after 1952. It may be due to China has established a medical insurance system for employees, including the public medical system in 1952 and the labor insurance medical system in 1953 [42, 43]. After the Second Domestic Revolutionary War, China launched a patriotic public health campaign, which greatly improved the health environment and public health, and also reduced the risk of TC . The increased risk of incidence in later-born men is related to social disintegration and later-born men are more likely to be exposed to smoking, drinking, and other risk factors that lead to higher risk levels. Complex interactions between these risk factors may lead to increasing TC risk of Chinese men . In the U. S., ultraviolet radiation may be one of the risk factors for TC. Because, since 1960s, the availability of medical services in the U.S., new equipment and complex diagnostic tests has increased, the number of X-rays for medical examinations and care has significantly increased . We found that people over the age of 40 are more likely to be exposed to ultraviolet radiation than younger people. It reflects that the cohort effect of TC is on declining trend . During 1913–1917 and 1993–1997, the incidence of TC for men and women showed a continuous downward trend in the U.S. The possible reason is that the recently born cohort is well-educated and has a better understanding of health and disease prevention .