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Cardiac Risk in Thyroid Treatment

Here are some findings from a literature search on this topic

1. In one study, doctors eight "young healthy subjects" 100mcg of T3 for 2 weeks! That's four times the dose we're usually considering.  And here's another 18 who got the same thing.

2. Staying mildly hyperthyroid caused only minimal cardiac changes.  One review seems to convey more concern.  One study compared mild hyperthyroidism, with less than obvious symptoms, to obvious hyperthyroidism, in terms of the effects of each state on the heart.  Mild hyperthyroidism did not show the heart changes seen in more severe states.

3. The frequency of worrisome heart rhythms (repetitions of ventricular beat changes) was the same in people who were quite hyperthyroid and normal controls.  

4. There is a single case report, among more than 200 articles on the subject of hyperthyroidism and the heart, of heart attack to due vessel "spasm" from hyperthyroidism. 

Conclusion: from the psychiatric literature (although minimal) and this cardiac literature, the use of low doses of T3 and T4 is acceptably safe, compared to the benefits which may result, even if mild hyperthyroidism is induced.  The greatest risk in such hyperthyroidism would be causing atrial fibrillation.