Here is a summary of a paper describing effects of pituitary failure on thyroid parameters:
L-T4 MONOTHERAPY IS NOT ADEQUATE FOR NORMALIZING SERUM FT3 IN MOST PATIENTS WITH CENTRAL HYPOTHYROIDISM
D. Gullo1, A. Latina1, I. Marturano1, A. Caff1, P. Tita1, G. Parrinello1, S. Squatrito1, R. Vigneri1
1Endocrinology, Department of Clinical and Molecular Biomedicine, Garibaldi-Nesima Hospital - Catania
Context. In central hypothyroidism (CH) the treatment of choice, as in primary hypothyroidism, is L-T4 because peripheral conversion to T3 is believed to account for the overall tissue requirement for thyroid hormones. In these patients TSH cannot be used to evaluate euthyroidism and FT4 is currently the most used measurement to adjust the replacement dose. In a large series of CH patients we evaluated whether L-T4 monotherapy can normalize serum thyroid hormones, and more specifically, the most active hormone FT3. Methods. In a cross-sectional retrospective study we measured FT3, FT4 and FT3/FT4 ratio in 76 CH patients, median age 58 yrs (IQR 37-69), M=33 F=43, all having normal FT4 serum levels under L-T4 monotherapy (reference range 9.0-20.6 pmol/L). Data were compared to those observed in a cohort of euthyroid controls (n=3,875). Results. In L-T4-treated CH patients median FT4 levels (14.0 pmol/L) was not different from those of euthyroid controls (13.2 pmol/L; P=0.60) whereas median FT3 was significantly lower (3.11 pmol/L vs 4.47; P< 0.001). In L-T4-treated CH patients, 97.4% had FT3 levels below the median FT3 of controls and 35.5% had FT3 lower than the reference range (2.93-6.01 pmol/L). The FT3/FT4 ratio, an index of peripheral deiodination, was 0.23 vs 0.32 (P< 0.001). FT3 and FT4 levels were influenced by gender, age and the treatment with other pituitary hormones. In 26 CH patients having serum FT4 in the highest tertile, suggested as the target value for optimal L-T4 therapy, FT3 levels were below the median FT3 of reference range in all cases and below 2.5 percentile of the normal range in 8/26 cases (30.8%). Conclusions. CH patients on L-T4 monotherapy have FT3 levels significantly lower than euthyroid controls, despite normalization of FT4. Most important, in one third of cases, even when FT4 is in the higher range, FT3 is often lower than the reference range, indicating that these patients’ ability to convert T4 to T3 is insufficient. The accompanying pituitary and peripheral hormone deficiencies may play a role in the reduced FT4 to FT3 conversion. These data indicate that FT4 alone is not sufficient for assessing TH status in CH patients. L-T4 monotherapy may not adequately guarantee euthyroidism in a significant subgroup of CH patients that may require a more physiological treatment with combined L-T4 and L-T3 administration.