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) 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.