Final answer:
The hematocrit is the most useful component of the CBC for determining a hyperthyroidism client's fluid status, as it measures the percentage of RBCs in blood and reflects hydration levels.
Step-by-step explanation:
When assessing the fluid status of a client with hyperthyroidism, the most useful component of the complete blood count (CBC) would be the hematocrit. Hematocrit measures the percentage of red blood cells (RBCs) in the total blood volume and can indicate hydration levels; a high hematocrit may suggest dehydration as the plasma volume falls, causing RBCs to take up a greater proportion of the blood volume. This measure can vary with the client's hydration status, with a lower percentage potentially indicating overhydration and a higher percentage indicating dehydration.