Resistance to Thyroid Hormone Diagnosed by the Reduced Response of Fibroblasts to the Triiodothyronine-Induced Suppression of Fibronectin Synthesis*

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Abstract
The syndrome of generalized resistance to thyroid hormone (GRTH) is due to a defect at the level of target tissues, but its diagnosis is currently based on indirect tests and cumbersome clinical observations. We recently reported that physiological amounts of T3 inhibit fibronectin (Fn) synthesis by human fibroblasts derived from normal donors. The aim of this study was to assess if this observation could be used for the direct tissue diagnosis of GRTH. Skin fibroblasts from seven normal subjects and seven patients with GRTH were grown to confluence and then exposed for 3 days to medium supplemented with 10% thyroidectomized bovine serum with or without added T3. Cells were then labeled with [35S]methionine for 4 h, and the combined medium and cell lysate was examined for the incorporation of [35S]methionine into trichloroacetic acid and anti-Fn-precipitable material, followed by analysis on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. 35S activity in the 230K protein was corrected for trichloroacetic acid-precipitable 35S activity. To assess the selectivity of the T3 effect on Fn in fibroblasts from normal subjects compared to GRTH patients, the responses of Fn to dexamethasone and Na butyrate were also determined. Addition of 10–9 and 10–7m T3 to the medium (free T3, 6 × 10–12 and 2 × 10–9m, respectively) reduced Fn synthesis by 13.2 ± 3.2% (±sd) and 14.3 ± 8.2%, respectively, in fibroblasts from patients with GRTH, an effect significantly less (P < 0.02 and P < 0.005) than the 22.2 ± 7.4% and 30.2 ± 5.0% reduction that occurred in fibroblasts from normal subjects. With the exception of one patient with GRTH, individual values from each of the two groups did not overlap; the clinical studies suggested that this patient had a milder form of GRTH. In contrast, dexamethasone and sodium butyrate stimulated Fn synthesis by 25.6 ± 8.7% and 268 ± 125%, respectively, in fibroblasts from patients with GRTH compared to 27.6 ± 7.2% and 304 ± 125% in fibroblasts from normal subjects. The difference of the responses between the two groups was not significant. The results indicate that measurement of the response of Fn to T3 in cultured fibroblasts can be used for the tissue diagnosis of GRTH.

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