Thyroid Research Thyroid Research Archive Hypothyroidism

Thyroxine inhibits thyrotropin secretion in patients with pituitary disease

(November 2002)

The background of the study. Patients with hypothyroidism caused by thyrotropin (TSH) deficiency (central hypothyroidism) have normal or low serum TSH concentrations as well as low serum thyroxine (T4) concentrations. The effects of T4 treatment in these patients are usually assessed by measuring serum free T4, rather than serum TSH, as is done in patients with hypothyroidism caused by thyroid disease (primary hypothyroidism).

How the study was done. The study subjects were 41 patients with hypothalamic or pituitary disease, low serum free T4 concentrations, and low, normal, or slightly high serum TSH concentrations. The patients were treated with 50 to 150 µg of T4 daily. The initial dose was increased gradually until the patients were euthyroid and had normal serum free T4 concentrations. Serum free T4 and TSH (assay sensitivity, 0.005 mU/L) were measured at multiple times in each patient during a mean follow-up period of seven years. The results were compared with similarly obtained results in 17 patients with primary hypothyroidism.

The results of the study. The mean pretreatment serum TSH concentration in the patients with central hypothyroidism was 2.0 mU/L; five patients had values >4.0 mU/L, and 9 had values <0.5 mU/L. During T4 treatment, 92 percent of patients who had serum TSH concentrations <0.1 mU/L had normal serum free T4 concentrations, as compared with only 34 percent of patients who had serum TSH concentrations >1 mU/L. The mean pretreatment serum TSH concentration in the patients with primary hypothyroidism was 75.5 mU/L, and the final concentration was 5.4 mU/L. Plots of the relationships between serum TSH and serum free T4 concentrations before and during treatment in the patients with central hypothyroidism and in the patients with primary hypothyroidism were parallel, but at any serum free T4 concentration the serum TSH concentrations were much lower in the patients with central hypothyroidism.

The conclusions of the study. Serum TSH concentrations decrease during T4 therapy in patients with central hypothyroidism, as in patients with primary hypothyroidism, but the concentrations are much lower at all serum free T4 concentrations.

The original article. Shimon I, Cohen O, Lubetsky A, Olchovsky D. Thyrotropin suppression by thyroid hormone replacement is correlated with thyroxine level normalization in central hypothyroidism. Thyroid 2002;12:823-7.