What Causes Spike In TSH Level Only During A Specific Month?

My yearly bloodwork is every January and my TSH is always at 7.43. I ususally get restested a month later and it goes down to about 5.2 and stays like that for the whole year. I gained on the medication so I chose not to take it. In July I was like 2.4. WHy does it rise so much in January? SHoudl I rtake medication? All my other tests are normal, cholesterol, T3, T4 etc. I dont have dry skin, just 8 lb. weight gain and morning fatigue