Which of the following can be a side effect of drug treatment for hyperthyroidism?

In the context of treating hyperthyroidism, drug treatments, such as antithyroid medications, aim to reduce the production of thyroid hormones. Increased heart rate, or tachycardia, is a common symptom associated with hyperthyroidism itself. As treatment progresses, it may alleviate this symptom, but during the initial phases of therapy or if dosages are not optimized, patients may still experience fluctuations in heart rate. This side effect can stem from the body's response to the changes in hormone levels as they stabilize.

The other options relate to symptoms or side effects that are not typically associated with the medication used for hyperthyroidism. Weight gain is often seen in conditions of hypothyroidism rather than hyperthyroidism. Decreased bone density is more commonly a concern with long-term untreated hyperthyroidism rather than a direct effect of drug treatment, and estrogen deficiency symptoms are unrelated to the pharmacological management of hyperthyroidism. These factors highlight the unique relationship between the treatment of hyperthyroidism and its symptoms, reinforcing that increased heart rate is indeed a relevant potential side effect during treatment.