The thyroid gland produces thyroxine (T4) and triiodothyronine (T3), hormones that govern the basal metabolic rate, bone growth, neural maturation, the body’s sensitivity to catecholamines, and the metabolism of proteins, fats and carbohydrates. Because these hormones touch nearly every physiological system, even a modest postoperative shortfall can translate into fatigue, weight gain and diminished quality of life. The gland consists of a right and a left lobe joined by a thin bridge of tissue called the isthmus, and in most patients the two halves are not equal partners. Earlier ultrasound studies had already established that the right lobe is typically the larger of the two; one frequently cited measurement from 2009 put the right lobe at 6.8 milliliters on average against 5.7 milliliters for the left. The Austrian research team behind the new study reasoned that this anatomical imbalance should, in principle, leave patients with less residual hormone-producing tissue after a right-sided resection—and therefore a greater need for pharmaceutical replacement.

To test that hypothesis, the investigators combed a hospital-wide research database for adults who had undergone hemithyroidectomy at the division of endocrine surgery and subsequently attended a follow-up visit at the thyroid outpatient clinic of the endocrinology department. From an initial pool of 459 patients, they excluded 33 who had undergone additional surgery on the contralateral lobe, leaving a final cohort of 426 individuals, 54.2 percent of whom had lost the right lobe and 45.8 percent the left. The team deliberately screened out anyone with prior thyroid surgery, radiation therapy, pre-existing hypothyroidism, autoimmune thyroid disease or pregnancy, all of which could have independently distorted hormone requirements. Preoperative thyroid volumes were calculated from ultrasound images using the standard volumetric ellipsoid formula, multiplying height, width and depth by a correction factor of 0.524.

The cohort was predominantly female—75.4 percent of patients—a distribution the authors note is expected given women’s well-documented susceptibility to thyroid disease. Baseline characteristics were strikingly well matched between the two surgical groups: mean age hovered around 53 years in both, and average height and weight were virtually identical. Preoperative TSH values, the most sensitive biochemical marker of thyroid status, were also comparable. That symmetry matters, because it strengthens the argument that any divergence in postoperative hormone needs can be attributed to the surgery itself rather than to differences in the patient populations. Overall, 91.3 percent of patients in the study developed hypothyroidism after their operation, a figure the authors acknowledge is likely inflated by the referral pattern of the outpatient clinic, which concentrates symptomatic patients.

The central result was unambiguous. Patients who had undergone right-lobe removal were taking a mean levothyroxine dose of 68.7 micrograms per day, compared with 60.6 micrograms per day after left-lobe removal, a statistically significant difference. When the researchers applied a multiple linear regression model with heteroskedasticity-robust standard errors, adjusting for the two strongest known confounders—body weight and preoperative TSH—the adjusted mean doses were 68.1 versus 60.2 micrograms per day, corresponding to the 13.2 percent relative increase. Expressed per kilogram of body weight, the median dose was 0.840 micrograms per kilogram per day after right-sided surgery against 0.773 after left-sided surgery. Consistent with the dosing gap, postoperative TSH was also higher in the right-lobe group, at a median of 1.70 versus 1.40 micro-international units per milliliter, indicating that even on their prescribed doses, these patients were running slightly more hypothyroid than their left-lobe counterparts.

The anatomical explanation held up in the imaging data. Among the 112 patients for whom preoperative ultrasound volumes of the resected lobe were available, the removed right lobes were on average 30 percent larger than the removed left lobes, a difference that itself reached statistical significance. This aligns with earlier work showing that the volume of the remaining lobe predicts hypothyroidism risk: a previous study found that patients whose contralateral lobe volume, adjusted for body surface area, fell below 3.2 milliliters faced roughly three times the risk of developing hypothyroidism after surgery. What the new analysis adds is the link from tissue volume all the way through to the practical prescription pad—the actual microgram dose a patient ends up taking months later.

Subgroup analyses added nuance without undermining the main finding. Among patients operated on for goitre, the adjusted dose difference rose to 16.0 percent, or an additional 9.37 micrograms per day, and remained statistically significant. In the group treated for nodules and autonomously functioning adenomas, the adjusted difference was 9.7 percent, or 6.0 micrograms per day, but did not reach significance, possibly reflecting the smaller subgroup size and the difficulty of cleanly separating patients who had both pathologies. An exploratory Spearman correlation analysis further showed that levothyroxine dose was associated with the side of surgery, preoperative TSH and body weight, whereas sex and age had no measurable impact. In the fully adjusted regression model, all three significant predictors held: surgical side contributed 7.69 micrograms per day, each kilogram of body weight added 0.38 micrograms per day, and higher preoperative TSH was associated with 7.65 micrograms per day.

The authors are candid about the limitations inherent in a retrospective design. The cohort represents patients who both had surgery and returned for follow-up at the same institution’s thyroid clinic, which likely selected for patients with postoperative symptoms and helps explain the high observed prevalence of hypothyroidism. The number and precise timing of levothyroxine dose adjustments made before the follow-up visit were not systematically recorded, so the titration process itself could not be reconstructed. Underlying conditions prompting the surgery, such as goitre or nodules, may also have influenced results, and many patients probably had both. Still, the study’s strengths are considerable: a large sample, a remarkably long observation period spanning nearly a quarter century, and rigorous exclusion of any patient with surgery on the contralateral lobe, which protects the side-specific comparison from contamination.

The practical implications could reshape how clinicians counsel patients facing thyroid lobectomy. Roughly 22 to 60 percent of all hemithyroidectomy patients currently require thyroid hormone supplementation despite the compensatory growth of the remaining lobe, which can expand by up to 30 percent of its original size. Existing prediction models have incorporated preoperative TSH, sex, age, body surface area and body mass index, and some researchers have argued that weight alone is the best single predictor of postoperative levothyroxine needs. The new data suggest that adding a single question—which lobe is being removed—could meaningfully sharpen those predictions. The authors propose that a preoperative ultrasound to size the remaining lobe may be especially worthwhile when right-lobe resection is planned, and they call for randomized, prospective studies to confirm the finding and integrate surgical side into future dosing algorithms. Until then, the message for patients and physicians alike is that in thyroid surgery, left and right are not interchangeable.

Subject of Research: Postoperative levothyroxine dosage requirements after right- versus left-lobe hemithyroidectomy

Article Title: Higher Post‐Operative L‐Thyroxine Dosage Needed After Right Compared to Left‐Lobe Hemithyroidectomy

Article References: Deischinger, C., Krebs, M., Scherer, T., Scheuba, C., Kautzky‐Willer, A., & Kosi‐Trebotic, L. (2026). Higher Post‐Operative L‐Thyroxine Dosage Needed After Right Compared to Left‐Lobe Hemithyroidectomy. Endocrinology, Diabetes & Metabolism, 9(5), Article e70345. https://doi.org/10.1002/edm2.70345

Image Credits: AI Generated

DOI: 10.1002/edm2.70345

Keywords: thyroid surgery, hemithyroidectomy, levothyroxine, hypothyroidism, TSH, thyroid hormones, endocrinology, thyroid lobes, retrospective cohort study, hormone replacement, ultrasound, thyroxine dosage