Audit of hyperthyroxinaemia and thyrotoxicosis using a sensitive TSH assay.

The aim of this study was determine (a) the causes of hyperthyroxinaemia and (b) the biochemical profile of thyrotoxicosis in a general hospital laboratory for one year using a sensitive TSH assay. Total T4 (TT4) and TSH were measured in all 8,382 samples and TT3, free T4, free T3 and thyroxine binding globulin (TBG) in selected cases. TT4 was elevated in 215 (2.6%). 159 (74%) were due to thyrotoxicosis; 41 (19%) to elevated TBG and 15 (7%) non-thyroidal illness. Thyrotoxicosis (serum TSH less than 0.15m U/1) occurred in 223 (2.7%) of all patients and was diagnosed with high TT4 in 159 (71%), normal due to intercurrent illness. 352 (4%) patients had suppressed TSH while all thyroid hormone values were normal. Thus TT4 may be elevated from causes other than thyrotoxicosis sufficiently frequently to necessitate routine TSH measurements. While Normal TSH measurements nearly always excludes thyrotoxicosis, suppressed values are insufficient to establish a diagnosis or monitor thyroxine replacement therapy.