In order to fully answer this question, we must define topsoil. Topsoil is very simply the top layer of the soil profile and can vary greatly depending on the type of vegetation, client, subsoil, and land use.
Now, with the definition of topsoil fresh in our minds we can examine loam. Loam is a subcategory of topsoil. Therefore loam is topsoil, but topsoil is not always loam. It is a mixture of sand, silt, clay, and organic matter. A medium loam has a makeup of 40% sand, 40% silt, and 20% clay according to the USDA Textural Triangle below (figure 1). Soil organic matter varies a great deal depending on the soil. In general organic content is less than 10% in naturally occurring soils (and most of the modified ones too). At D&H Loam we normally make additions of compost if the organic content is too low in a soil that we intend to sell. Our regular screened topsoil falls into the sandy loam or loam category and we try to stay as consistent as possible when we look for materials.