Hi, Thanks for your question. Please rate my answer if you are satisfied. First of all, your mom is 88 and she only started thyroxine 2 weeks ago. That means she might be having undiagnosed or untreated hypothyroidism for a long time. L-thyroxine is an analogue of the natural thyroid hormone and it can cause anxiety, irritability, agitation and anger. If your mom is having these effects she should have the dose adjusted and lowered. The reason simply could be that the dose of L-thyroxine is more than she needs or if the dose is fine for her, her body is over-responding to the normal dose which is likely because of two reasons: one that with advancing age the hormone receptors become over sensitive to normal hormone concentrations, two, when your body has a low level of anything for a long duration of time it adjusts to that level, then when you give it even the normal amount of that substance the body behaves as if it was too much. Your mom s L-thyroxine dosage needs to be adjusted she should visit back her endocrinologist.