Thyroid resistance to TSH complicated by autoimmune thyroiditis

In this report we describe a 47-yr-old woman who was referred to our department for elevated serum TSH associated with normal free thyroid hormone levels, suggesting subclinical hypothyroidism. When first seen she was clinically euthyroid, and her thyroid gland was normal in size both at palpation and by ultrasound. The ultrasound of the thyroid showed a normoechogenic pattern. Serum thyroid hormone levels were confirmed to be within the normal range, whereas the serum TSH concentration was moderately elevated (13.4 μU/ml). Tests for antithyroperoxidase, antithyroglobulin, and anti-TSH receptor antibodies gave negative results. The only son of the proband, a clinically euthyroid 23-yr-old man, had a slightly elevated serum TSH concentration (5.2 μU/ml) with normal free thyroid hormone levels. The entire coding regions of the TSH receptor gene were sequenced in the proband, the son, and the father of the son. Genetic analysis in the proband showed a homozygous inactivating mutation of the TSH receptor. The mutation consisted of the substitution of an alanine in place of proline at position 162 in the extracellular portion of the receptor. The son was heterozygous for ProAla. Only the wild-type sequence was found in the father. Both the proband and her son were considered to have compensated TSH resistance and were not treated. After 2 yr of follow-up, new thyroid tests were performed in the proband and showed a marked increase in the serum TSH concentration (61 μU/ml) compared with the initially observed value; serum free T and T levels were in the low normal range. At that time, tests for antithyroglobulin and antithyroperoxidase antibodies gave positive results, and thyroid echography showed a gland of normal size, but with a diffuse hypoechogenic pattern. In conclusion, we describe the first case of compensated TSH resistance evolving to mild hypothyroidism due to the appearance of a chronic autoimmune thyroiditis.