Levothyroxine
Levothyroxine is a manufactured copy of thyroxine, the main hormone the thyroid releases; the body converts it into the more active form as needed. It replaces what an underactive thyroid no longer makes, restoring the pace of metabolism, body temperature, heart rate and energy. It is used in hypothyroidism, usually for life.
Absorption is the point of all the instructions: it goes on an empty stomach in the morning, with a gap before breakfast and before the first coffee, since both cut how much gets in. Iron and calcium supplements bind it in the gut and belong several hours away, and omeprazole, esomeprazole and pantoprazole lower stomach acid enough to reduce absorption as well. Small differences between manufacturers can shift how much reaches the blood, which is why staying with the same brand matters, and TSH shows whether replacement is at the right level, with free T4 added in some situations.
Palpitations, a racing or irregular pulse, tremor, sweating, sleeplessness and weight loss suggest more hormone than the body needs; returning tiredness, feeling cold, constipation and weight gain suggest too little. Either pattern, and any chest pain, is a reason to see a doctor rather than to change the drug alone.
Reference information, not medical advice. Discuss any change with your doctor.