Thyroid hormone indices during illness in six hypothyroid subjects rendered euthyroid with levothyroxine therapy

We wanted to evaluate changes in the natural course of serum thyroxine (T4), tri-iodothyronine (T3), reverse tri-iodothyronine (rT3), and thyroid stimulating hormone (TSH) concentrations during hospitalization for an acute illness, in subjects rendered euthyroid with Levothyroxine (LT4) replacement therapy.

Six male subjects ranging in age 30 - 65 years with a history of primary hypothyroidism were included. They were euthyroid prior to hospitalization. LT4 continued to be administered orally in the same pre-admission daily dose. Serum, T4, T3, rT3, and TSH concentrations were determined on day of admission to the intensive care unit (ICU) for an acute illness. These were repeated during the first week on alternate days and again during a follow-up visit 1 week after discharge. Student's t-test, analysis of variance, and linear regression were used to analyze the data.

Serum T4, T3 declined to a nadir and serum rT3 rose to its peak by day 3 of hospitalization before returning to pre admission euthyroid levels. Serum TSH declined initially but rose to supernormal levels on day 7 before normalization. Significant correlations were noted between TSH on one hand and T3/T4 (r = 0.76, p < 0.001) and rT3/T4 (r= - 0.64, p < 0.001) ratios.

Alterations ensuing during a short stay in the hospital due to an acute illness in subjects with primary hypothyroidism rendered euthyroid with appropriate replacement therapy with Levothyroxine (LT4) are almost identical to those in normal subjects. These changes are probably secondary to altered thyroid hormone metabolism. The altered levels of thyroid hormones and TSH noted in these subjects are transient and therefore providers should refrain from initiating frequent changes in daily LT4 replacement dose during the acute illness in these subjects.