Diagnosing hypopituitarism from suggestive thyroid function tests
DJ Preiss & LM Todd
The typical pattern of thyroid function tests associated with secondary hypothyroidism, partial hypopituitarism and panhypopituitarism consists of subnormal free-T4, normal or marginally elevated TSH and normal T3 concentrations. Only laboratories offering frontline TFTs including fT4 and TSH, rather than TSH alone, can identify such cases. Our prospective study assessed the number of cases of hypopituitarism which would be diagnosed by reflective testing on such samples at our hospital which serves a population of 200 000.
Over 15 months, suitable samples were identified from TFT results using the laboratory information system. Confirmed cases of hypopituitarism, repeat samples and lost or insufficient specimens were excluded from further analysis. Clinical information and other results were used to identify cases of non-thyroidal illness and specific thyroid conditions (antithyroid medication, post thyroidectomy on T3). The remaining samples had fT4 reanalysed following assay recalibration. Confirmed subnormal fT4 lead to the addition of relevant pituitary and endocrine assays (gonadotrophins, cortisol, prolactin, total T3, testosterone).
315 results were identified from 73 650 TFT results. Following exclusions of the relevant samples, review of clinical information and other results, and appropriate reflective testing, 10 new cases of hypopituitarism were identified. This represents an incidence of 40 cases per million per year. 8 patients are currently attending endocrine clinics. Pituitary imaging was abnormal in 3 cases (large pituitary tumour, macroadenoma, empty sella), normal in 2 cases and 3 patients currently await MRI scans. Isolated secondary hypothyroidism could not be excluded in a further 32 cases.
Hypopituitarism is more common than previously reported and may be identified by recognition of TFT patterns. fT4 should therefore be an integral part of frontline TFTs as TSH alone is unable to assist in the identification of possible hypopituitarism. Laboratories should have a protocol for investigation of suggestive TFTs.