Biochemical hypothyroidism (BH) after hemithyroidectomy is an under-recognized complication with a reported incidence of 9% to 43%. This study aimed to identify potential clinicopathologic risk factors associated with early (<12 months after hemithyroidectomy) and late-onset (≥12 months after hemithyroidectomy) BH.

From 2005 to 2008 there were 263 postsurgical patients who were eligible for analysis. Serum thyroid stimulating hormone (TSH) level was checked regularly after surgery. Postoperative TSH reaching a level higher than 5.5 mIU/L was defined as BH. The overall median follow-up period was 21 months (range, 3–62 mo). Any clinicopathologic factors significantly associated with BH in the univariate analysis were entered into multivariate analysis. A further analysis was performed comparing factors between early and late-onset BH.

There were 38 patients who developed subsequent BH, 33 of these cases developed within 2 years. Those patients with BH were significantly older (P = .037), had a higher preoperative TSH level (P < .001), longer follow-up period (P < .001), more frequent thyroiditis on histology (P = .043), lighter resected tissue weight (P = .001), and were more likely to have positive antimicrosomal antibodies (P = .043) than those without BH. However, in the multivariate analysis after adjusting for different follow-up periods in the 2 groups, only lighter resected tissue weight (P = .036) and concomitant thyroiditis on histology (P = .005) turned out to be independent factors for BH. Thyroiditis on histology was also the only significant risk factor for developing early onset BH.

Patients with lighter resected tissue weight and concomitant thyroiditis on histology were particularly at risk for subsequent BH. Although not all patients with thyroiditis developed BH, in those who did develop BH it occurred within the first 11 months.