What investigation should be arranged for a couple who have not conceived after 24 months of trying?

In the scenario of a couple who have been trying to conceive for 24 months without success, the priority investigation typically involves evaluating potential causes of infertility in both partners, particularly assessing the male partner's reproductive health. A semen analysis is a critical initial test because it provides essential information about sperm count, motility, and morphology, which are pivotal factors in male fertility.

While testing for chlamydia can be important in ruling out sexually transmitted infections that can affect fertility, it often comes into play alongside more direct assessments of fertility. Testing for testosterone levels and thyroid function might be necessary based on specific clinical concerns but are not first-line investigations for infertility after such a prolonged period of trying to conceive.

Focusing on the semen analysis aligns with established infertility work-up protocols, making it the most pertinent choice in this context. By performing this analysis first, healthcare providers can identify any male factor infertility issues that may need to be addressed, guiding further management and treatment options.