Persistence of low serum thyroid hormone levels in a Graves' disease patient receiving supraphysiologic L-thyroxine replacement therapy.

A patient with Graves' disease was treated with radioactive iodine. For several years following treatment, the patient displayed clinical hypothyroidism and persistently low serum thyroxine (T4) and triiodothyronine (T3) levels despite large T4 replacement dosage (0.3-0.4 mg L-thyroxine daily). A defect in T4 absorption was considered unlikely since absorption of fat soluble materials (vitamins A and E) was essentially normal as reflected by their serum concentrations. Abnormalities in serum protein binding of T4 especially by immunoglobulins were suspected; however, thyroid hormone binding antibodies were absent. Thyroxine binding prealbumin (TBPA) levels were either frankly elevated or in the upper normal range and such variations were mirrored by retinol binding protein (RBP) concentrations. Thyroxine binding globulin (TBG) concentration was normal. A surprising finding was an elevated percent dialyzable thyroxine (.041%; normal range, .018-.034%) in spite of a normal concentration of TBG. Serum free fatty acid levels were also elevated. The marked increase in percent free T4 (FT4) fraction together with a low serum total T4 concentration resulted in normal or marginally elevated FT4 levels. An increase in T4 metabolic clearance as suggested by the elevated percent FT4 fraction was corroborated by steady state serum T4 values observed following changes in T4 dosage.(ABSTRACT TRUNCATED AT 250 WORDS)