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Serum thyroid hormone concentrations fall in thyroxine-treated patients during acute nonthyroidal illness

(November 2004)

The background of the study. Serum thyroxine (T4) concentrations fall during nonthyroidal illness in some patients, a change that could indicate a decrease in T4 secretion, a decrease in serum protein binding of T4, or an increase in uptake of T4 into tissue. Serum T4 and other thyroid hormones and thyrotropin (TSH) were measured repeatedly during a nonthyroidal illness and after recovery in patients with hypothyroidism who were treated with T4.

How the study was done. The study subjects were six men with hypothyroidism taking 0.1 to 0.15 mg of T4 daily who had an acute illness and were hospitalized on an intensive-care unit. All had normal serum T4, triiodothyronine (T3), and thyrotropin (TSH) concentrations before the onset of their illness. They were given their usual dose of T4 while hospitalized. Serum total T4, total T3, and TSH were measured at the time of admission (day 1); on days 3, 5, and 7; and 7 days after discharge.

The results of the study. The average serum T4 and T3 concentrations were lower on day 1 than at the time of pre-illness testing (Table). The values were even lower on day 3, and then gradually increased. The average serum TSH concentration was lower on day 1 than at the time of pre-illness testing, decreased further on day 3, then increased to above both the pre- and post-illness values on day 7.

Table. Average Serum Thyroid Hormone and TSH Concentrations in T4-Treated Men before, during, and after an Acute Nonthyroidal Illness.
Pre-Illness Day 1 Day 3 Day 5 Day 7 Post-Illness
Serum T4 (µg/dl) 8.4 4.2 4.1 5.8 7.2 8.6
Serum T3 (ng/dl) 136 73 56 91 109 132
Serum TSH (mU/L) 2.1 0.6 0.4 1.7 4.6 2.3

The conclusions of the study. Serum T4 and T3 concentrations fall transiently during acute illness in patients with hypothyroidism being treated with T4. Since the patients were taking T4, the changes were probably due to decreased serum protein binding of T3 and T3.

The original article. Wadwekar D, Kabadi UM. Thyroid hormone indices during illness in six hypothyroid subjects rendered euthyroid with levothyroxine therapy. Exp Clin Endocrinol Diabetes 2004;112:373-7.

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Nonthyroidal Illness