What was your Free T4/Free T3 when your TSH was at the 0.18 level or did your doctor adjust your medication based only on the TSH level? TSH is a pituitary hormone, not a thyroid hormone and usually is not indicative of actual thyroid status when one is on thyroid replacement medication.
1) Lightheadedness can be a symptom of hyperthyroidism or over medication. I haven't experienced it, personally, but over the 13 yrs I've been on this forum, we've had people who have.
2) We've also had people who have been on the same dose for many years, then suddenly it's no longer suitable for them. There's no one explanation, except that our bodies can change over time, making our medication requirements change as well. If you've lost weight, changed your diet or medications or changed the way or timing of your medication, you might need a different dose.
3) High thyroid hormone levels can cause rapid heart rate, palpitations, etc. Therefore, a lower dose of Synthroid would, presumably, lower your thyroid hormone levels, which would in turn lower your heart rate.
Hi, I want to provide an update so that others might benefit from my experience. After dealing with being lightheaded since July 2020, I am starting to turn the corner. My TSH/FT4 and FT3 were all showing hyper levels. The doctor reduced by Synthroid from 112 to 88. All the recent blood work came back normal after the dosage adjustments...by the way it took 6 months so please keep that in mind and stay patient. Along the way, I read that hyper and hypo can contribute to iron deficiency. Many of the symptoms I had were very similar to low iron. So I stress the following:
* Check Serum Ferritin levels if you have corresponding symptoms of fatigue, dizziness , lightheaded, cold hands/feet and fast heartbeat., chest pain, etc.
Although I am going to get my serum ferritin levels checked soon, I started to feel 100% improvement after taking iron supplements for two weeks. In retrospect, my symptoms have been gradually building over many years. My primary care only tested for TSH (which was low normal) but I could have been clinically hyper for quite some time. The knock on effect (in my case the iron) can be hard to pick up. Always test for FT4 and FT3!
Hope this helps.