Hypothyroidism usually responds to levothyroxine, a synthetic form of T4, usually required for life and subject to regular testing. Levothyroxine is also suitable for subclinical hypothyroidism but TSH levels need proper assessment.[^7,11]

While generic prescribing of levothyroxine is appropriate for most patients, a small proportion may still report thyroid dysfunction symptoms or start to experience side effects if the brand or manufacturer is changed. MHRA guidance recommends trying to supply such patients consistently with an acceptable formulation if possible (testing for disease changes if necessary) or to even consider an oral solution.[²¹]

NICE advises against using the synthetic form of T3, liothyronine, due to insufficient evidence of any benefits over levothyroxine monotherapy. However, for some people, a small change in the gene expressing the D2 enzyme may reduce available T3, so a patient may see some benefit from having liothyronine and levothyroxine co-prescribed.[^7,22,23]

Women with hypothyroidism who are planning a pregnancy or become pregnant should have additional blood tests to determine thyroid activity and to change the dose of levothyroxine. If not, the risk of complications increases, including pre-eclampsia (high blood pressure), birth defects, premature birth or miscarriage.[¹¹]

Liothyronine is not recommended for pregnancy. As it will not pass through the placenta, the developing foetus may be at risk of not developing properly due to not being exposed to sufficient thyroid.[²⁴]