Serum thyroid hormone indexes in patients with primary hyperparathyroidism.

Serum total reverse triiodothyronine (rT3) levels are normal in patients with renal diseases with and without renal insufficiency but elevated in nonrenal nonthyroidal illnesses. To evaluate the role of secondary hyperparathyroidism of renal diseases in this difference, serum thyroid hormone levels were studied in 27 patients with primary hyperparathyroidism (PHP) and normal renal function. In PHP, total T3 levels were reduced (118 +/- 6 ng/dL, normal: 147 +/- 3 ng/dL) and correlated with PTH levels. Serum rT3 levels were also decreased (27 +/- 3 ng/dL, normal: 34 +/- 2 ng/dL). Values for serum total thyroxine (T4), T3 uptake ratio, free T4 index, and thyrotrophin were not altered. Serum rT3 levels were increased (63 +/- 13 ng/dL) in patients with hypercalcemia due to malignant neoplasms who had low T3 levels, undetectable PTH and normal renal function. Thus, PTH excess may be the factor responsible for the failure of rT3 levels to increase in PHP and secondary hyperparathyroidism.