From Longman Dictionary of Contemporary Englishthinningthin‧ning /ˈθɪnɪŋ/ adjective someone with thinning hair is losing their hair
Examples from the Corpus
thinning• The hair was already thinning and perhaps to compensate he had grown a luxuriant Groucho moustache which almost hid his mouth.• He took encore after encore until the thinning crowd finally disappeared into the dark autumn evening.• He stared at the old stooped man with the thinning grey hair and bushy walrus moustache who controlled the nation's money.• He was thick-set, with thinning hair brushed back, a magnificent walrus moustache and several missing teeth.• Unfortunately, there is no miracle cure for thinning hair but there are some very good treatments around.• The eyes had sunken in and the skull was showing through the thinning hair of the pate.• The porter joined them, scratching his thinning hair. 