Three in Ten Patients Develop Hypothyroidism Within Six Months of Hemithyroidectomy

When surgeons remove half of the thyroid gland, they often reassure patients that the remaining lobe will carry the hormonal load alone. A new retrospective cohort study from Saudi Arabia suggests that this reassurance deserves a caveat: nearly three in ten patients whose thyroid function was entirely normal before surgery developed hypothyroidism within six months of a hemithyroidectomy, and the strongest predictor was not the size of the tumor, the type of nodule, or the presence of autoimmune antibodies, but a single preoperative number — the level of thyroid-stimulating hormone circulating in the blood.

The study, led by Ryan Abdullah Alharbi of Imam Abdulrahman Bin Faisal University in Dammam and colleagues, followed consecutive patients who underwent hemithyroidectomy at a university hospital between August 2016 and February 2026. Of 191 patients reviewed, 159 had usable thyroid-function data, and 128 of those were euthyroid — meaning their thyroid function was normal before the operation — forming the primary cohort for analysis. Every patient in that primary cohort had at least two postoperative measurements of thyroid-stimulating hormone, or TSH, within the six-month follow-up window, giving the researchers a dense biochemical picture of how the remaining lobe adapted, or failed to adapt, to working alone.

The headline result is striking: 38 of the 128 euthyroid patients — 29.7 percent, with a 95 percent confidence interval of 22.5 to 38.1 percent — developed new-onset hypothyroidism within six months. Yet the character of that hypothyroidism matters as much as its frequency. The overwhelming majority of cases, 97.4 percent, were subclinical, meaning TSH rose above the normal range while free thyroxine, the active thyroid hormone, remained within normal limits. Only 2.6 percent of cases were overt, with both TSH elevated and thyroxine depressed. Even so, the subclinical label can be deceptive: 21.1 percent of the affected patients reached a peak TSH of at least 10 micro-international units per milliliter, a threshold that in many guidelines triggers consideration of treatment even when symptoms are absent. Six additional patients showed transient TSH elevations that did not meet the criteria for sustained hypothyroidism, hinting at a spectrum of postoperative adaptation rather than a binary outcome.

The physiology behind this pattern is well understood, even if its incidence has been inconsistently documented. The thyroid operates as a feedback loop: the pituitary gland secretes TSH in proportion to how much circulating thyroid hormone it detects. When half of the gland is removed, the remaining lobe must compensate, and its capacity to do so depends on how much functional reserve it retains. A lobe already working near its ceiling — signaled by a higher preoperative TSH, which indicates the pituitary is already asking the gland to work harder — has less headroom when suddenly asked to double its output. Age compounds the problem, likely through gradual replacement of functional thyroid tissue with fibrous tissue and a blunted secretory response over time.

The numbers bear this out with unusual clarity. Patients whose preoperative TSH was at least 2 micro-international units per milliliter — still within the laboratory normal range, but at the higher end — developed hypothyroidism at a rate of 56.8 percent, compared with just 18.7 percent among those with lower preoperative TSH, an unadjusted odds ratio of 5.71. In a multivariable logistic regression model that adjusted for both variables simultaneously, each 1-unit increase in preoperative TSH carried an adjusted odds ratio of 2.263 (95 percent CI 1.436 to 3.565, p < 0.001), and each additional decade of age carried an adjusted odds ratio of 1.440 (95 percent CI 1.003 to 2.069, p = 0.048). Both associations held independently, externally validating relationships established in other populations within a contemporary Saudi cohort.

Just as revealing is what did not predict the outcome. Sex, nationality, comorbidities, the TI-RADS ultrasound classification of the nodule, the Bethesda category of any preoperative biopsy, the histopathological diagnosis, and the presence or absence of thyroid antibodies all failed to show a statistically significant association with postoperative hypothyroidism. That null result is clinically meaningful: it suggests that the routine preoperative workup — imaging, cytology, pathology — does not identify which euthyroid patients will lose thyroid reserve after lobectomy. The biochemical baseline, not the anatomical or pathological findings, carries the prognostic weight.

The study also broke from much of the prior literature by including patients with pre-existing thyroid dysfunction, a group often excluded from earlier analyses. Among 19 patients who were hypothyroid before surgery, TSH rose after hemithyroidectomy and their levothyroxine doses had to be up-titrated, with the median dose increasing from 50 to 91 micrograms (p = 0.004) — a substantial escalation reflecting the reduced functional reserve of a gland already dependent on hormone replacement. Among 12 patients who were hyperthyroid before surgery, TSH rose toward normal (p = 0.021), a direction of change that is physiologically expected: removing half of an overactive gland reduces the overall secretory capacity, pulling the feedback loop back toward equilibrium. These subgroups were small and exploratory, but they illustrate that postoperative thyroid dynamics differ fundamentally depending on the preoperative state.

The practical implications are straightforward. The authors conclude that routine biochemical monitoring is warranted after hemithyroidectomy regardless of preoperative thyroid status — not just for patients with known dysfunction, but for every patient, because the preoperative TSH value identifies risk even among those considered normal. A patient with a TSH of 2.5 before surgery and a normal free thyroxine may be told the operation is low-risk, yet carries nearly a threefold higher odds of developing hypothyroidism than a patient with a TSH of 1.0. Monitoring should therefore be framed not as a precaution for the vulnerable few but as standard follow-up for all, with particular vigilance for older patients and those at the higher end of the normal TSH range.

Published in BMC Endocrine Disorders as an open-access article, the study adds a contemporary Saudi data point to a global conversation about the long-term consequences of thyroid lobectomy, a procedure increasingly favored over total thyroidectomy for unilateral nodules because it preserves function and avoids lifelong hormone replacement in many patients. The finding that roughly one in three euthyroid patients will still require monitoring — and that a meaningful fraction will reach TSH levels prompting treatment consideration — tempers the narrative that lobectomy is uniformly the gentler option. It does not reverse that narrative, since most patients retain normal function, but it sharpens the preoperative conversation: the remaining lobe is not a guarantee, and the preoperative TSH is the best available clue to how well it will cope alone.

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

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

Article References: Alharbi, R. A., AlShehri, F. D., Alwohaibi, I. N. I., Alnajran, S. A., Alzuabi, O. M., Omar, A. K. A., & Almajid, F. M. (2026). Incidence and factors associated with hypothyroidism after hemithyroidectomy: a retrospective cohort study in Saudi Arabia. BMC Endocrine Disorders. https://doi.org/10.1186/s12902-026-02655-7

Image Credits: AI Generated

DOI: 10.1186/s12902-026-02655-7

Keywords: Incidence, factors, associated, hypothyroidism, hemithyroidectomy, retrospective, cohort, Saudi, Arabia, scientific research