Chronic Blood Loss: This is likely due to menstrual disorders, gastrointestinal bleeding, or other chronic blood loss conditions. A hemoglobin (Hgb) level of 16 is slightly elevated, which could be a response to chronic blood loss as the body tries to compensate for the loss of red blood cells by producing more.

Dehydration: Dehydration can cause a relative increase in hemoglobin concentration due to decreased plasma volume. In a 67-year-old female, dehydration could be due to various reasons such as inadequate fluid intake, diarrhea, or other medical conditions.
Polycythemia Vera: Although less common, polycythemia vera, a myeloproliferative disorder, could lead to an elevated hemoglobin level. However, it would typically be associated with other symptoms and findings such as thrombocytosis and leukocytosis.
Chronic Hypoxia: Conditions leading to chronic hypoxia, such as chronic obstructive pulmonary disease (COPD) or sleep apnea, can stimulate erythropoietin production, leading to increased red blood cell production and elevated hemoglobin levels.

Renal Cell Carcinoma: Although rare, renal cell carcinoma can produce erythropoietin, leading to secondary polycythemia. Given the potential severity of this condition, it's crucial not to miss it, despite its low likelihood.
Testosterone Replacement Therapy: Exogenous testosterone can stimulate erythropoiesis, leading to elevated hemoglobin levels. This is particularly relevant if the patient is undergoing hormone replacement therapy.

Erythropoietin-Producing Tumors: Besides renal cell carcinoma, other rare tumors can produce erythropoietin, leading to elevated hemoglobin levels. These include hepatocellular carcinoma, phaeochromocytoma, and others.
High-Altitude Adaptation: Living at high altitudes can lead to increased erythropoietin production and, consequently, higher hemoglobin levels as an adaptation to lower oxygen levels. However, this would be relevant only if the patient recently moved to a significantly higher altitude.