Side effects
Liothyronine may cause a number of side effects, mostly similar to symptoms of hyperthyroidism, which include:
weight loss
tremor
headache
upset stomach
vomiting
diarrhea
stomach cramps
nervousness
irritability
insomnia
excessive sweating
increased appetite
fever
changes in menstrual cycle
sensitivity to heat

Boxed warning
The package insert contains the following boxed warning, as do all thyroid hormones:

Drugs with thyroid hormone activity, alone or together with other therapeutic agents, have been used for the treatment of obesity. In euthyroid patients, doses within the range of daily hormonal requirements are ineffective for weight reduction. Larger doses may produce serious or even life-threatening manifestations of toxicity, particularly when given in association with sympathomimetic amines such as those used for their anorectic effects. Pharmacology
Liothyronine is the most potent form of thyroid hormone. As a salt of triiodothyronine (T3), it is chemically similar and pharmacologically equivalent to T3. As such, it acts on the body to increase the basal metabolic rate, affect protein synthesis and increase the bodys sensitivity to catecholamines (such as adrenaline) by permissiveness. As monotherapy or in combination therapy with SSRIs, liothyronine may also enhance generation of new neurons in the central nervous system. The thyroid hormones are essential to proper development and differentiation of all cells of the human body. These hormones also regulate protein, fat, and carbohydrate metabolism, affecting how human cells use energetic compounds. In comparison to levothyroxine (T4), liothyronine has a faster onset of action as well as a shorter biological half-life, which may be due to less plasma protein binding to thyroxine-binding globulin and transthyretin.
Association for Genitourinary Medicine (UK) and the Medical Society for the Study of Venereal Diseases (UK), 2001. External links

Zoons Balanitis at eMedicine
Dermatological atlas