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 greater than 12 microU/ml (normal, 0.3-5.7 microU/ml), with FT3I greater than 85 (normal, 70-160) and FT4I greater than 7 (normal, 4.2-11). Four others had TSH greater than 6 microU/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 greater than 4.2 by day 8 and FT3I greater than or equal to 70 by day 18, but serum TSH concentration did not fall consistently below 20 microU/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.(ABSTRACT TRUNCATED AT 250 WORDS)