Checkpoint inhibitors have gained increased traction in recent years as they have improved prognosis in various malignancies. Pembrolizumab, an anti-programmed cell death protein (PD-1) monoclonal antibody, has become a first-line chemotherapeutic agent for stage II non-small cell lung cancer since 2019. Although much more common with nivolumab, several immune-related adverse effects, particularly endocrinopathies, have been linked with pembrolizumab. We describe a case of a 59-year-old man with a history of unspecified lung cancer who presented with severe hyponatremia later attributed to secondary adrenal insufficiency and accompanying primary hypothyroidism secondary to pembrolizumab. Diagnosing adrenal insufficiency in patients on immune checkpoint inhibitors like pembrolizumab can be challenging due to nonspecific symptoms, making it crucial to rule out other causes of hyponatremia. Immunotherapy is known to cause thyroid immune-related adverse events, and anti-thyroid antibodies may not always be present in the diagnosis of hypothyroidism. Although there are some reported cases of pembrolizumab-induced adrenal insufficiency, the link between immunotherapy and endocrine disorders remains unclear. To our knowledge, no case reports exist that describe both primary hypothyroidism and secondary adrenal insufficiency after taking pembrolizumab, although such cases have been documented with axitinib. Timely diagnosis and treatment of adrenal insufficiency is crucial to prevent adverse effects, especially in patients with cancer receiving immunotherapy, as highlighted in this case.