The relationship between thyroid antibody titer and levothyroxine dose in patients with overt primary hypothyroidism.

BACKGROUND: Both excess and insufficient thyroid hormone replacement may produce adverse effects in various target tissues; therefore, understanding factors that affect achievement of target TSH levels is crucial.

OBJECTIVE: Investigate the relationship between antibody titers and levothyroxine dose.

DESIGN: Retrospective, review of data in medical records.

SETTING: Thyroid center of Fatih Sultan Mehmet Education and Research Hospital, Istanbul, Turkey.

PATIENTS AND METHODS: The study population consisted of patients that had been diagnosed as having overt primary hypothyroidism and were taking levothyroxine for at least one year. The serum TSH level for an euthyroid state was between 0.5-4 mIU/L. The levels of anti-thyroid peroxidase (TPOAb) considered positive for antibodies were < 5.6 IU/mL and for anti-thyroglobulin (TgAb) autoantibodies < 4.10 IU/ mL.

MAIN OUTCOME MEASURE: Daily levothyroxine doses of antibody-positive and negative patients and association of daily drug requirement with antibody titers.

RESULTS: The study population consisted of the 303 patients (273 females and 30 males with the mean [SD] age of 46.6 [13.2] years). In the antibody-positive group (n=210) average daily levothyroxine dose was statistically significantly higher than in the antibody-negative group (n=93) (mean of 78.8 [36.7] vs 64.2 [27.1] mg/day, P=.001, respectively). There was a low but statistically significant positive relationship between the TPOAb (r=0.217, P < .01) and TgAb levels (r=0.158, P < .05) and levothyroxine doses in the antibody-positive group.

CONCLUSION: Antibody titers are positively associated with larger levothyroxine (LT-4) replacement dosing in patients with autoimmune thyroiditis.

LIMITATION: Unknown antibody titers before starting levothyroxine use.