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.
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