Improvement of Laboratory Diagnostics in Hypothyroid Patients Using Levothyroxine

Hypothyroidism is a thyroid disorder and one of the most common endocrine disorders. Hypothyroidism can have multiple causes; most patients suffer from primary autoimmune hypothyroidism (Hashimoto's disease), but also central hypothyroidism, hypothyroidism after total thyroidectomy due to thyroid carcinoma, or hypothyroidism due to therapy of Graves' disease occur. Most patients with hypothyroidism are treated with levothyroxine (L-T4) to supplement the lack of thyroxine (T4) produced by their own thyroid. Serum thyroid-stimulating hormone (TSH) and/or free T4 (fT4) are currently measured to assess the efficacy of this therapy and to establish euthyroidism. It is known that fT4 concentrations in patients using L-T4 can be above the upper limit of the reference interval, while their TSH is not (completely) suppressed. This raises the question whether fT4 is an accurate reflection of thyroid hormone status in patients using L-T4. TSH is considered a reliable parameter of thyroid hormone status; however, TSH cannot be used to assess thyroid function in specific hypothyroid patient groups (e.g. central hypothyroidism). Free triiodothyronine (fT3), the active thyroid hormone, has been suggested to be an interesting alternative of fT4 to assess thyroid function. Previously, the methods to measure fT3 were not that robust; however, methods to determine fT3 have been improved, are currently reliable and not susceptible to changes due to L-T4 intake. In addition, the fT3/fT4 ratio is thought to be an interesting candidate in assessing thyroid hormone status as well. The aim of this study is to improve laboratory diagnostics of thyroid hormone status in patients with hypothyroidism receiving L-T4 in whom TSH cannot be used as a reflection of thyroid hormone status. We will primarily investigate the additional already available laboratory tests fT3 and fT3/fT4 ratio. We hypothesize that treated hypothyroid participants who are assumed euthyroid based on TSH (e.g. patients with Hashimoto's hypothyroidism) but have fT4 concentrations above the upper reference limit will more often have a fT3 level or a fT3/fT4 ratio within the reference interval. Concentrations of alternative markers in healthy controls and patients with Hashimoto's hypothyroidism with 'normal' TSH concentrations can, thus, be used to predict thyroid hormone status in patients using L-T4 in whom TSH cannot be used to assess thyroid hormone status.