Endocrine Abstracts

The effect of growth hormone (GH) replacement on thyroid function in hypopituitary patients has only been studied in small groups of children and adults with conflicting results.

We aimed to define the effect and clinical significance of adult GH replacement on thyroid status in a large cohort of 243 patients with hypopituitarism due to various causes. 84 patients were considered euthyroid before GH treatment and 159 patients had central hypothyroidism and were receiving thyroxine replacement. All patients had severe GH deficiency and the GH dose was titrated over 3 months to achieve serum IGF-1 concentration in the upper half of the age-adjusted normal range.

In the euthyroid group, we observed a mean reduction of serum free thyroxine (FT4) concentration of 2.6 pmol/l (P<0.001) and mean increase of total triiodothyronine (TT3) of 0.09 nmol/l (P=0.06) but a non-significant change in serum TSH. 30/84 patients (36%) became hypothyroid and needed initiation of thyroxine therapy. Patients who became hypothyroid had lower baseline serum FT4 concentration (13.9±3.3 pmol/l) compared with patients who remained euthyroid (16.3±3.3 pmol/l), P=0.02 and were more likely to have multiple pituitary hormone deficiencies (44% vs 16% respectively, P=0.01). In patients receiving thyroxine at baseline, we observed a mean reduction in FT4 of 1.2 pmol/l (P=0.11) and mean increase in TT3 of 0.1 nmol/l (P=0.017). 25/159 patients (16%) needed an increase in thyroxine dose.

GH deficiency masks central hypothyroidism in a significant proportion of hypopituitary patients and this is exposed after GH replacement. We recommend that hypopituitary patients with GH deficiency and serum free thyroxine concentration below 14 pmol/l should be considered for thyroxine replacement.