Here are some findings from a literature search on this topic
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 from this cardiac literature, the therapeutic use of thyroid hormone — as long as it is causing no symptoms of hyperthyroidism — appears to carry relatively little risk, compared to the benefits which may result. If it does not have clearly positive results, which should be very impressive and not easily obtained with other more standard treatments, then it should be tapered down until thyroid testing shows normal-range results.
The questions should be:
a) How much risk is there in trying this approach, if it doesn’t work for mood? (short-term exposure)
My opinion: it appears that in people who as far as they know have normal hearts, that risk is very low. If standard treatments have not worked, the potential for benefit can be high. Everyone has to work out the trade-off for themselves, with their physician.
b) How much risk is there in staying on thyroid at more than physiologic doses (thyroid tests are in the “hyperthyroid range)? (long-term exposure)
My opinion: first, we have to admit that not enough is known. If the benefits are spectacular and the treatment options are few (or if they have their own risks, often also rather unknown, e.g. if the medicine is less than 10 years old), it might be worth it. Again, everyone has to work out the trade-off for themselves, with their physician.