Introduction: Identification and treatment of central hypothyroidism in hypopituitarism is crucial particularly in patients with significant lethargy under consideration for GH replacement. Early Identification of TSH deficiency can be challenging particularly when both the TSH and fT4 lie within the normal range.
Methods: 26 362 TFTs derived from ambulatory community dwelling individuals represented the control group. TFTs from 227 patients with a putative or established insult to the hypothalamo-pituitary axis were studied; 145 from patients with central hypothyroidism on levothyroxine replacement and 82 samples from patients with hypopituitarism without a diagnosis of central hypothyroidism.
Results: Median fT4 (5th and 95th percentiles) value for the control group was 14.3 (10.919.5) pmol/l. Hypopituitary patients without a formal diagnosis of secondary hypothyroidism showed median fT4 13.1 (9.516.5) pmol/l, significantly lower than in the control group (P<0.001). Of these individuals 11% showed fT4 levels below the 5th percentile of the control group. Graphically, a left shift of the fT4 distribution curve was observed in this subgroup. The degree of hypopituitarism, defined by the number of pituitary hormone deficits, had no effect on the median fT4 level within this subgroup. In hypopituitary patients with a diagnosis of secondary hypothyroidism, and established on levothyroxine replacement, the median fT4 was 16.3 (11.123.7) pmol/l representing significantly higher values than observed in the control group (P<0.001). Graphically, the distribution curve for these individuals showed a wider base and lower peak frequency.
Conclusion: These data suggest that a subtle decrease in fT4 is present in a relatively large proportion of patients who receive a putative insult to the hypothalamopituitary axis. In the majority of these individuals fT4 values remain well within the normative range. Furthermore, examination of fT4 values of patients on levothyroxine shows difficulties with placement of the fT4 and potential over-treatment in a number of individuals.