[Effects of long-term administration of theophylline on the pituitary-thyroid axis in asthmatics]

The effects of long-term administration of theophylline on the pituitary-thyroid axis were investigated. In the first study, the changes in serum thyroid hormone, TSH and cortisol levels were examined before and after 2 week-administration of sustained-release theophylline in 5 normal and 10 asthmatic subjects. In the normal subjects, the hormone levels did not change before or after theophylline administration. On the other hand, in the asthmatics, serum TSH levels increased (p less than 0.05) and serum free T3 levels decreased (p less than 0.05) after 2 weeks. In the asthmatics, basal T4 levels had a significant correlation with both basal T3 and rT3 levels, but T4 levels after 2 weeks correlated significantly with rT3 alone. In the second investigation, the effects of long-term administration were studied in 15 asthmatics not on theophylline (group 1), 20 asthmatics on theophylline (group 2), and 20 asthmatics on theophylline and glucocorticoids (group 3). TSH levels in group 2 were significantly higher than those in groups 1 and 3. Although in group 1 there was a significant correlation between T4 levels and T3 and rT3 levels, in group 2 T4 levels correlated significantly with rT3 levels alone. In conclusion, these results suggest that although long-term theophylline treatment may increase the secretion of TSH from the pituitary in asthmatics, in the peripheral tissue it may decrease the conversion of T4 to T3, resulting in the diminution of active hormones.