Thyroid function tests (TFTs) are amongst the most commonly requested laboratory 
investigations in both primary and secondary care. Fortunately, most TFTs are 
straightforward to interpret and confirm the clinical impression of 
euthyroidism, hypothyroidism or hyperthyroidism. However, in an important 
subgroup of patients the results of TFTs can seem confusing, either by virtue of 
being discordant with the clinical picture or because they appear incongruent 
with each other [e.g. raised thyroid hormones (TH), but with non-suppressed 
thyrotropin (TSH); raised TSH, but with normal TH]. In such cases, it is 
important first to revisit the clinical context, and to consider potential 
confounding factors, including alterations in normal physiology (e.g. 
pregnancy), intercurrent (non-thyroidal) illness, and medication usage (e.g. 
thyroxine, amiodarone, heparin). Once these have been excluded, laboratory 
artefacts in commonly used TSH or TH immunoassays should be screened for, thus 
avoiding unnecessary further investigation and/or treatment in cases where there 
is assay interference. In the remainder, consideration should be given to 
screening for rare genetic and acquired disorders of the 
hypothalamic-pituitary-thyroid (HPT) axis [e.g. resistance to thyroid hormone 
(RTH), thyrotropinoma (TSHoma)]. Here, we discuss the main pitfalls in the 
measurement and interpretation of TFTs, and propose a structured algorithm for 
the investigation and management of patients with anomalous/discordant TFTs.