Background: Expanding ultrasound use has increased the detection of thyroid cancer. Incidence has always been higher among females, a disparity that has grown over time. The sex difference in thyroid cancer is understudied in the context of diagnostic testing, particularly among privately insured adults in whom thyroid cancer is most common. We evaluated the association between thyroid ultrasound, fine needle aspiration biopsy (FNAB), and cancer incidence by sex in a large, integrated health system. Methods: This longitudinal retrospective cohort study included Kaiser Permanente of Washington enrollees aged 18 and over who underwent thyroid ultrasound from 1997 to 2019. Data included electronic billing claims for patients linked to tumor registry diagnoses. We estimated (1) annual overall ultrasound, FNAB, and cancer incidence rates; (2) the proportion of ultrasound requiring FNAB; and (3) cancer diagnoses per FNAB. A Poisson model with offset determined the relationship between sex and the proportion of ultrasound requiring FNAB adjusting for patient and sociodemographic characteristics. Results: A total of 33,589 patients underwent ultrasound (78% females; mean age 56). Ultrasound rates per 100,000 covered lives, defined as insured individuals per year, increased five-fold among males (111.11-490.97) and >four-fold among females (382.27-1331.14) between 1997 and 2019. FNAB rates also increased over time (rates per 100,000: 174.09-430.37 in females vs. 58.38-189.13 in men). Overall, FNAB rates per ultrasound changed little over time, and FNAB per ultrasound was greater in males compared with females (Adj rate ratio = 1.06 [confidence interval 1.01-1.11]). Cancer incidence was higher in females over the study period, but cancer incidence per FNAB was similar between sexes (both 0.06, p = 0.4). Conclusions: Sex disparities in thyroid ultrasound rates are stark and are a likely driver of sex disparities in thyroid cancer incidence. Interestingly, ultrasound-triggered FNAB was more common in males and changed little over time, challenging the prevailing understanding that females have much higher rates of thyroid cancer. Although the population-based differences between sexes for FNAB and cancer were large, the differences among people who had ultrasound were small.
Keywords: health services research; sex disparities; thyroid cancer; ultrasound.