Thyrotropin and thyroid hormone economy in euthyroid Hashimoto's thyroiditis

Little is known about thyrotropin (TSH) and thyroid hormones in euthyroid Hashimoto's thyroiditis (HT), thus the aim was to investigate TSH and thyroid hormone economy in euthyroid HT and its relation to thyroid function. Ninety-five patients with euthyroid HT with normal TSH and thyroid hormones on the last follow up between 2009 and 2011 were investigated. Previous observation period ranged from 1.5 to 4.8 (mean 2.8) years, and they had never been treated with levothyroxine. The results of TSH and thyroid hormones were compared with 210 healthy subjects and expressed as median (25%-75%). According to TSH value, the subjects were divided into quartiles: TSH 0.4-0.99 (1q), 1.0-1.99 (2q), 2.0-2.99 (3q) and 3.0-4.0 mIU/L (4q). Euthyroid HT patients had higher TSH (2.53 [1.79-3.14] vs.1.95 [1.24-2.72], p < 0.001). T4 and T3 were not different. The distribution of TSH in HT patients was significantly shifted to the right; 71% of patients were in the 3q and 4q groups. When HT patients with higher TSH (3q and 4q) were compared with those with lower TSH (1q and 2q), significant differences emerged in TSH (3.01 [2.48-3.48] vs.1.45 [1.07-1.71] mIU/L), T4 (99.0 [88.2-112.0] vs.112.0 [105.0-122.0] nmol/L) and T3 (1.78 [1.48-2.05] vs. 2.10 [1.85-2.21] nmol/L; p < 0.01). TPO values were similar in both groups. A gradually increasing proportion of euthyroid HT patients with at least one supranormal TSH during the observation period were found: 0% in 1q, 10% in 2q, 15% in 3q and 44% in 4q TSH group. Euthyroid HT patients maintain euthyroidism only under strenuous TSH stimulation. The patients with high normal TSH are identified as those with a major risk of hypothyroidism in the near future.