Endocrine Abstracts

The TSH B/I ratio is normally reduced overnight and is substantially reduced in patients with overt central hypothyroidism due to h-p axis tumours before and after treatment. It is unknown if similar changes in the TSH B/I ratio in the absence of overt central hypothyroidism contribute to the increased TSH levels seen in cancer survivors who undergo h-p axis irradiation. Thus, we selected 9 patients with the highest TSH levels out of a previously reported cohort of 37 euthyroid adult cancer survivors cranially irradiated for non-pituitary brain tumours or leukemia, in whom the mean TSH of the 24 hourly samples during 24-hour profiling and the peak stimulated TSH levels during the TRH test were increased. Of the 9 patients, 4 had both fed and fasting (last 24 hours of 33-hour fast) profiles, 2 had fed and 3 had fasting profile only. Pooled day (0900–2000 hr) and night (2100–0800 hr) sera were analysed for the TSH B/I ratio. Patients had significantly higher profile mean TSH and significantly lower free T4 levels compared with 33 normal controls (mean±S.E.M.; 3.15±0.2 vs 1.8±0.1 mU/l; P<0.0001 and 13.7±0.9 vs 15.5±0.4 pmol/l; P=0.04, respectively). The ratio (mean±S.E.M.) in the patients was not different from that reported in 26 controls (1.8±0.2 vs 1.5±0.1; P=0.2). The daytime and night-time ratios in the patients were also similar to those reported in 16 controls (2±0.3 vs 2±0.06; P=0.8 and 1.6±0.4 vs 1.3±0.1; P=0.2, respectively). Paired observations in 4 patients revealed no impact of fasting on the ratio (2.2±0.2 vs 2.1±0.5; P=0.8). We conclude that, in the absence of overt central hypothyroidism, h-p irradiation does not alter the TSH B/I ratio.