Which of the following medications requires blood work before administration?

Thryotabs, which contain the active ingredient levothyroxine, is a medication used to treat hypothyroidism in dogs and cats. Prior to administering this medication, it is crucial to confirm the diagnosis of hypothyroidism through blood tests that measure thyroid hormone levels (specifically T4 and possibly TSH levels) to ensure that the animal is indeed deficient in thyroid hormone. This is important not only for confirming that the condition is present but also for determining the appropriate dosage of the medication based on the severity of the deficiency and the individual patient's needs.

Other medications listed, such as Gabapentin, Amoxicillin, and Aspirin, do not typically require blood work prior to their administration in routine cases. Gabapentin is often used for pain management or seizures and does not necessitate blood tests to start treatment. Amoxicillin is a broad-spectrum antibiotic used for bacterial infections and typically doesn't require blood work before its use. Aspirin, while an analgesic and anti-inflammatory, is also used without the need for prior blood tests under normal circumstances.