Hypothyroidism is one of the commonest endocrine disorders, affecting about 3% of the adult population. Levothyroxine (T4) is the standard treatment for hypothyroidism and is one of the most commonly prescribed drugs in the UK. Whilst most patients with hypothyroidism are satisfied with this treatment, a small subgroup of patients do not feel well on T4 monotherapy. Whether such patients would benefit from Liothyronine (T3), taken either in combination with T4, as a monotherapy or as a component of desiccated animal thyroid extract, remains controversial. This controversy has further escalated in recent years due to a huge increase in the price of T3 in the UK, and wide variations in the clinical practice and policies of clinical commissioning groups across the country in relation to the prescription of T3. Several randomised control trials and subsequent meta-analyses have failed to show a clear-cut benefit of T3T4 combination therapy over T4 monotherapy. However, limitations of these clinical trials include small sample sizes, relatively short follow-up periods, and use of variable and non-physiological dosage of T3. The recently published draft NICE guidelines on thyroid diseases recommend not to routinely offer T3 for primary hypothyroidism, either alone or in combination with T3, because of not enough evidence that it offers benefits over T4 monotherapy. This presentation will discuss evidence for and against the use of T3 in subgroup of patients with hypothyroidism.