A reduction in brain function, as indicated by 18F-FDG uptake, is observed in which condition?

In the context of brain function as indicated by 18F-FDG (fluorodeoxyglucose) uptake in PET scans, reduced uptake correlates with decreased brain activity and metabolism. Alzheimer's Disease is a condition characterized by neurodegeneration, leading to significant reductions in glucose metabolism in specific brain regions. This reduction is distinctly observable on PET scans, where areas like the parietal and temporal lobes typically show decreased 18F-FDG uptake in affected individuals, indicating impaired brain function.

While hypothyroidism, hyperthyroidism, and diabetes mellitus may have various effects on the body and can influence brain health, they do not exhibit the same patterns of widespread reduction in brain glucose metabolism as observed in Alzheimer's Disease. In the case of hyperthyroidism, increased metabolic activity may actually enhance glucose uptake in the brain rather than reduce it. Therefore, the most direct link between reduced brain function, as demonstrated through diminished 18F-FDG uptake, is specifically tied to Alzheimer's Disease, making it the correct answer.