Evaluation of a Laboratory Reflex Testing Protocol to Detect Hypopituitarism in Primary Care presenting as Hypothyroxinaemia

Background Thyroid function tests (TFTs), are routinely requested by general practitioners (GPs) in the clinical biochemistry laboratory. Hypothyroxinaemia (low fT4) accompanied by TSH within the reference interval (RI) 1 is a discordant pattern which is seen commonly in non-thyroidal illness and also as result of medications. Hypopituitarism is a lot rarer, but a serious condition the laboratory does not want to miss. Methods All thyroid hormone samples from primary care meeting the discordant case definition under investigation [fT4<10 pmol/L and TSH within RI (0.3-4.2 mU/L)] had partial anterior pituitary profiles [PAPP (cortisol, oestradiol/testosterone, prolactin, gonadotrophins)] added as reflex tests and results interpreted by a chemical pathologist. From January to June 2023, we conducted structured interviews with the requesting GPs, and, where indicated, requested repeat samples for full anterior pituitary profile [FAPP (PAPP, growth hormone (GH) and insulin-like growth factor 1 (IGF-1)]. We also reviewed the laboratory records of patients with previously known hypopituitarism to determine their fT4 and TSH values at diagnosis. Results Over the 6 months 41,487 GP TFTs were requested; 54 (0.13%) fitted the discordant case definition and had PAPP reflexed.13 FAPPs were requested. We identified 3 cases of hypopituitarism. The number of additional tests required to diagnose 1 case of hypopituitarism was 129. In 74% of reflex-tested cases, there was a plausible explanation for the TFT pattern (medications, known thyroid dysfunction, non-thyroidal illness, pregnancy). Conclusion This study highlights the importance of medical liaison and early intervention in a biochemistry laboratory in identifying cases of unsuspected hypopituitarism.