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Serum TSH Concentration as an Aid to Monitoring Compliance with Thyroid Hormone Therapy in Hypothyroidism

To monitor the compliance of patients taking levothyroxine as replacement therapy for primary hypothyroidism, the authors measured serum thyroid-stimulating hormone (TSH), free thyroxine index (FT4I), and free triiodothyronine index (FT3I) in patients attending the endocrine clinic. During a 6-month period, there were 159 visits by 132 patients with treated hypothyroidism. Five of the 132 patients had TSH levels > 12 μU/ml (normal, 0.3–5.7 μU/ml), with FT3I > 85 (normal, 70–160) and FT4I > 7 (normal, 4.2–11). Four others had TSH > 6 μU/ml with normal FT3I and normal FT4I. These nine patients had normal thyroid tests documented on replacement therapy in the past. Five of the nine admitted to discontinuing their medicine for more than 1 week or taking it erratically. Two untreated hypothyroid patients had daily thyroid function tests after they were started on full replacement doses of levothyroxine. Both achieved FT4I > 4.2 by day 8 and FT3I ≥ 70 by day 18, but serum TSH concentration did not fall consistently below 20 μU/ml until day 23 in one patient and day 21 in the other and did not fall to the normal range by discharge at day 37 in one patient or day 42 in the other. Thus, although treatment with levothyroxine will normalize serum T4 and T3 concentrations within 3 weeks, normalization of serum TSH may take several more weeks. This prospective study in a large out-patient clinic shows that 4% of patients with treated primary hypothyroidism may have elevation of serum TSH to more than twice the upper limit of normal, while serum T4 and T3 are clearly normal. In the follow-up evaluation of patients taking stable replacement doses of levothyroxine for primary hypothyroidism, an elevated serum TSH may be an indication of noncompliance with therapy.