How come weight loss supplements dangerous for those with thyroid disease?

Caffeine: This question is more relevant to individuals with the diagnosis of hyperthyroidism. As you may already know, many of theses so called "diet" supplements contain caffeine or caffeine like ingredients that tend to raise heart rate. Those who have hyperthyroidism, especially if not treated or under treated, can be at risk of cardiac arrhythmias/fainting spells if large amount of caffeine ingested.

Stimulants: Stimulant drugs are often the main components of otc dietary supplements. Those with active hyperthyroidism may not be able to tolerate more stimulation to their bodies. Any dietary supplement should be approved by the a treating physician before taking it. Some supplements aren't particularly safe for anyone.