Modulation of thyroid parameters by exogenous thyroxine in familial dysalbuminemic hyperthyroxinemia

Abstract

A patient with familial dysalbuminemic hyperthyroxinemia (FDH) was given graded doses of exogenous thyroxine (0.2 mg/d for 2 weeks; 0.4 mg/d for 2 weeks; 0.6 mg/d for 2 weeks) to study modulation of various thyroid parameters. The plasma concentration of the serum transport proteins, thyroxine binding globulin (TBG), sex hormone binding globulin (SHBG), and cortisol binding globulin (CBG) as well as serum thyroxine (T 4), triiodothyronine (T 3), absolute free thyroxine (FT 4), and serum protein binding of T 4 tracer were measured. At the end of T 4 treatment, T 4 and T 3 were increased by 151% and 78%, respectively. The FT 4 increased (157%), while the percent dialyzable free T 4 fraction (DFT 4) showed no significant change. SHBG, a protein sensitive to thyroid hormone (TH) action, increased 148% (from 0.23 to 0.57 μg/dL) after treatment but this concentration was still in the normal range; TBG and CBG decreased by about 16%. Analysis of the electrophoretic 125I-T 4 distribution pattern in serum during T 4 treatment showed essentially no change in TBG-bound T 4 (percent tracer carriage X total T 4), while there was a progressive increase in albumin-bound T 4 (341% increase over pretreatment value) and a lesser increase in prealbumin (TBPA)-bound T 4 (187%). These observations describing alterations in TH action, serum T 4-protein binding, and the failure of percent DFT 4 to increase with elevation in serum total T 4 are of clinical significance in evaluating thyroid function parameters in FDH patients undergoing TH treatment.