Blunted nocturnal TSH surge does not indicate central hypothyroidism in patients after pituitary surgery^*

Summary

The thyrotropin-releasing hormone stimulation test (TRH test) is commonly used as part of the endocrine evaluation after pituitary surgery. However, some patients with a normal thyrotropin (TSH) response to TRH after pituitary surgery develop central hypothyroidism during follow-up. On the other hand, hypothyroidism does not necessarily ensue in patients with a blunted TSH response. As TSH is secreted in a pulsatile fashion with maximum secretion in the early morning, we investigated whether measurement of the nocturnal TSH surge is useful for predicting development of thyrotropic function after pituitary surgery. Serum TSH concentrations were measured at hourly intervals from 16.00 h to 06.00 h in 13 healthy volunteers and in 10 patients within 2 weeks after pituitary surgery. A standard TRH test using iv injection of 200 ug synthetic TRH was performed the next morning. Three and six months later thyroid function was reassessed in all patients by measuring thyroid hormones and TSH. Healthy volunteers showed a clear nocturnal TSH surge from a nadir of 0.55 ± 0.27 µIU/ml at 18.00 h to a peak concentration of 1.82 ± 0.97 μU/ml at 06.00 h (p = 0.0015). ATSH during TRH test was 6.31 ±2.27 μIU/ml. In contrast, following pituitary surgery, patients invariably showed a blunted nocturnal increase in TSH concentration, which was 0.27 ± 0.20 µIU/ml at 18.00 h and 0.33 ± 0.26 μIU/ml at 06.00 h (p = 0.044). ΔTSH during TRH test was 1.99 ± 2.51 μIU/ml and was subnormal in 8 out of 10 patients. Levothyroxine supplementation was initiated in two of these patients, because free T4 levels were also subnormal and clinical hypothyroidism was present. In the remaining patients with subnormal TRH response, no case of central hypothyroidism was identified at the follow-up visits after 3 and 6 months. We conclude from these data that both nocturnal TSH surge and TRH test are subnormal after pituitary surgery and do not indicate that central hypothyroidism will develop.

Key words

Hypothyroidism - pituitary tumors - thyrotropin - endocrine testing