For 131I therapy to be effective, the trace thyroid tissue must be avid to iodine, which is achieved by elevating the person's TSH levels.

Hypothyroid mothers should continue to take thyroid hormone replacement therapy throughout pregnancy to avoid adverse events.

[10][11] Breastmilk contains a low amount of thyroid hormone, so it is important to exercise caution when breastfeeding while taking liothyronine.

[10] TSH must be routinely monitored since there is a risk of coronary artery disease, hyperthyroidism and excessive bone loss from inadequate or abnormal thyroid replacement.

[12] Liothyronine may cause a number of side effects, mostly similar to symptoms of hyperthyroidism, which include:[13] The package insert contains the following boxed warning, as do all thyroid hormones:[12]

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.Liothyronine is the most potent form of thyroid hormone.

[citation needed] 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.

[citation needed] The British Competition and Markets Authority launched an investigation into the alleged "excessive and unfair pricing" of liothyronine tablets in 2017.