We had the same top suspicion; I like your way of thinking.
When I hear about a fractious (that is a titch on the grumpy side) cat who is hungry all the time, is losing weight and drinking and urinates a lot - hyperactive thyroid would be top on my list. Blood work is necessary for diagnosis - the other possibilities mention are also possible, although Cushing's is uncommon in cats (kidney disease, diabetes would be the other top concerns). If he is hyperthyroid, a blood pressure test would be very important as well.
The problem with sedation is that many hyperthyroid cats will have cardiac disease secondary to the thyroid disease. This may make any sedation associated with risk. Have your veterinarian make that decision and I would recommend against the use of any sedative use that you may have at home. Perhaps some homeopathic remedies may help - Lickables makes a tasty gel formula called Travel-Ease - it contains chamomile and L-tryophane. I would talk to your veterinarian about this however, prior to giving it as well.
Usually we can obtain the blood without sedation. However with that said, I have had to use some anesthetics on rare occasion to pull blood samples. Luckily the medication for hyperthyroidism can come in a tuna flavored liquid which will make medicating easier. Also as the thyroid comes under control, the fractious behavior will ease up. While many of the hyperthyroid cats seem to have always 'hated the vet' - a portion of the fractiousness is related to the hyperthyroidism. John Adams was thought to have had an overactive thyroid (Grave's disease) and it was thought that the disease may have lead to his temper.
Remember - talk to your vet about any homeopathic calming agents first. When the blood work comes back - if you have any questions, let us know.
Best of luck to you guys!