Get emergency medical help if you have signs of an allergic reaction: hives; difficult breathing; swelling of your face, lips, tongue, or throat.
Stop using liothyronine and call your doctor if you have symptoms of thyroid toxicity, such as chest pain, fast or pounding heartbeats, feeling hot or nervous, or sweating more than usual.
Also call your doctor at once if you have:
weak or shallow breathing;
confusion;
a feeling of being cold all the time;
changes in weight or appetite;
chest pressure spreading to your jaw or shoulder;
a light-headed feeling, like you might pass out;
severe headaches, ringing in your ears, dizziness, vision problems, pain behind your eyes;
a seizure; or
(in a child or teenager) pain, stiffness, and trouble moving the legs.
Common side effects may include:
trouble breathing;
headache;
tremors, feeling nervous or irritable;
muscle weakness;
increased appetite;
diarrhea;
irregular menstrual periods;
weight loss;
feeling hot;
rash; or
sleep problems (insomnia).
This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.
 Liothyronine side effects (more detail)
