Hi Parin

Quoting from this paper

One approach to increase power for GWASs on a phenotype that is hard to collect is to utilize an intermediate or proxy phenotype that is correlated to the target phenotype of interest. In this approach, one intermediate or proxy phenotype, which is highly correlated and easily collectable, is collected and then a GWAS is performed on the intermediate phenotype in order to detect associated signals. For example, triglyceride levels can be collected as a proxy for obtaining information about metabolic diseases. This approach is known as intermediate phenotype analysis.

Hope you find this useful

-Vijay