What Causes Skipped Heart Beats?
Thyroid
Detailed Answer:
Sorry to note his complaints.
I see you have mentioned that his thyroid numbers are fine. May I know if the thyroid labs were checked after he began the pale diet/ weight loss or before that.
Because as you have partially surmised, the thyroid dose may be excessive at the new weight he has achieved since thyroid dosing is often weight related. When the body gets more synthroid than it needs, one of the symptoms that may develop is an irregular heart beat/rhythm.
However, if the thyroid tests were ran after 4 to 6 weeks after the new diet and weight loss, and the TSH is in the range of 0.4 to 3.5 then it is extremely unlikely the synthroid is causing the skipped beats.
Amongst the medications you have mentioned, there is no significant interaction of these with synthroid to explain the skipped beats, especially if the the TSH and free T4 appear normal on the labs. Also make sure his blood (25 hydroxy) vitamin D levels are normal.