Incidence and factors associated with hypothyroidism after hemithyroidectomy: a retrospective cohort study in Saudi Arabia

Background Hypothyroidism after hemithyroidectomy is recognized, but Saudi evidence is limited, and prior literature has focused on euthyroid patients, leaving those with pre-existing dysfunction poorly characterized. We aimed to estimate the six-month incidence of new-onset hypothyroidism and identify associated factors in Saudi Arabia. Methods We conducted a retrospective cohort study of consecutive patients who underwent hemithyroidectomy at a university hospital between August 2016 and February 2026. Of 191 patients, 159 were analyzed after excluding those without usable thyroid-function data or whose only postoperative measurement followed completion thyroidectomy; 128 euthyroid at baseline formed the primary cohort, with 19 pre-existing hypothyroid and 12 hyperthyroid patients as pre-specified subgroups. All primary-cohort patients had at least two postoperative TSH measurements within six months. The primary outcome was new-onset hypothyroidism. Associations were tested univariably, and age and preoperative TSH were examined in a multivariable logistic model. Results New-onset hypothyroidism occurred in 38 of 128 patients (29.7%, 95% CI 22.5–38.1) within six months; 97.4% were subclinical and 2.6% overt, though 21.1% reached a peak TSH ≥ 10 µIU/mL; 6 further patients had transient elevations. Higher preoperative TSH and older age were significantly associated with the outcome; a TSH ≥ 2 µIU/mL was associated with a higher rate (56.8% vs. 18.7%; odds ratio 5.71). Both remained independent after adjustment (preoperative TSH: adjusted OR 2.263 per µIU/mL, 95% CI 1.436–3.565, p < 0.001; age: adjusted OR 1.440 per decade, 95% CI 1.003–2.069, p = 0.048). No statistically significant association was detected for sex, nationality, comorbidities, TI-RADS, Bethesda category, histopathology, or antibody status. Among preoperatively hypothyroid patients, TSH rose and levothyroxine was up-titrated (median 50 → 91 mcg; p = 0.004); among hyperthyroid patients, TSH rose toward normal ( p = 0.021). Conclusions Approximately three in ten euthyroid patients developed new-onset hypothyroidism within six months, most subclinical. Higher preoperative TSH showed the strongest association, and older age was also independently associated, externally validating established relationships in a contemporary Saudi cohort. Routine biochemical monitoring is warranted regardless of preoperative status. In a small exploratory subgroup with pre-existing hypothyroidism, levothyroxine doses were commonly escalated. Clinical trial number Not applicable.