TSH. The TSH will be high with hypothyroidism and low with hyperthyroidism.
History. No tests: if you are hyperthyroid, your engine is running hot and you have a fast heart beat, palpitations, sweats, tremors, are hot, and can have diarrhea. If you are hypothyroid, then you have slow heart rate, feel cold and sluggish and can be constipated. If I was to order one test, it would be a tsh. If it's high, then you are hypothyroid; if it's low then you are hyperthyroid.