T4 levels rising after 15 years stable post-thyroidectomy

[allwaysjudee]

Thirty years after radiation therapy for Hodgkins, my thyroid developed a goiter and had to be removed due to too many abnormal (pre-cancerous) cells. A tiny morsel was left behind, less than 1 cm. For 15 years, I've been on 125 mg Euthyrox, and all thyroid levels have remained stable until last year. TSH remains very low - 0.014 in 2015, when my T4 went up above normal, (with nervous like symptoms). Lowered Euthyrox to 100 mg, everything normalized, but now 18 months later, my T4 levels are high again -24 - while TSH remains low at 0.04. Multiple episodes of tachycardia this time, so the doc wants to lower Euthyrox to 75 mg. My main question is, what can be causing the rising T4 if my intake is unchanged? With such a low TSH, the tiny morsel remaining shouldn't be producing anything, so what else can cause the T4 to go up to the point it causes tachycardia? Thanks in advance for any info.