Pituitary-thyroid function in chronic renal failure assessed by a highly sensitive thyrotropin assay

Pituitary-thyroid function was assessed in 40 patients with chronic renal failure undergoing regular maintenance hemodialysis and in 35 normal subjects. Serum thyroid hormone levels were significantly lower in the dialysis patients than in the normal subjects (P less than 0.001) and were in the hypothyroid range in a high proportion of dialysis patients (total T3, 25%; free T3, 45%; total T4, 55%; free T4, 45%; and free T4 index, 38%). The reduced free thyroid hormone levels could not be explained by currently recognized assay artefacts. Serum TSH levels were higher than in the normal subjects (P less than 0.01), but still within the normal range for most (35 of 40) dialysis patients and did not correlate significantly with total or free thyroid hormone concentrations in either group. These results suggest some impairment in the thyroidal response to TSH and impaired pituitary response to low serum thyroid hormone levels, the latter implying resetting of the normal feedback mechanism such that diminished thyroid hormone production evokes a smaller than normal increase in TSH secretion. This diminished thyrotroph sensitivity to reduced serum thyroid hormone levels may be beneficial in severe nonthyroidal illness.