You will need an oral thermometer and a note pad and pen.
Place the thermometer, pad and pen on your night table by your bedside

Take an armpit (under the armpit for at least five minutes) temperature before going to bed, write it down.
Upon awakening and before getting out of bed - no bathroom or stirring around. Place the thermometer under the arm pit again for five minutes. Write the temperature down. Do this for at least 5 days. At the end of 5 days average the morning temperatures ( add all five temps together then divide by 5) and if lower than 97.8º - providing the night temp. is at least 98.7º  or above -  on a consistent basis it's a good indication that the thyroid is under functioning...  98.7 and above most likely hyperactive thyroid. But symptoms can help to absolutely determine the thyroid function.

This is simple test that Dr. Whitaker and a few other alternative practicing physicians has patients use to check the thyroid, and has been shown to be more accurate than thyroid function blood tests, which may not show that your thyroid is not functioning properly until it is really not working for quite sometime...

PS: My daughter's conventional lab Thyroid tests - ALL of them - said he thyroid was normal before, during and since her thyroid cancer!