Post-Levothyroxine Thyroid Dysfunction in Saudi Arabian Patients with Hypothyroidism: A Cross-Sectional Study

Background: Post-thyroxine treatment of thyroid dysfunction remains a clinical concern, especially in Middle Eastern populations. Methods: This descriptive cross-sectional study was conducted in 2023 at King Fahad Hospital, Hufof, Kingdom of Saudi Arabia. Of the 237 patients treated with L-thyroxine (L-T4) for hypothyroidism, 163 patients, almost exclusively females (152 females, 11 males), met the inclusion criteria and were enrolled. Thyroid hormones, lipid profiles, and 25-hydroxyvitamin D (25OH-D) were measured using standard laboratory assays. Results: Only 57% of patients achieved euthyroid status following L-T4 treatment, while 12.3% developed post-thyroxine-treatment (PTT) hyperthyroidism, and 30.7% developed PTT hypothyroidism. Older age was significantly associated with dysthyroidism (p = 0.018), whereas obesity (p = 0.937) and vitamin D levels (p = 0.982) were not. Total cholesterol (TC) and LDLc positively correlated with TSH levels, while elevated triglycerides (TGs) were significantly associated with PTT hyperthyroidism. The two dysthyroid subgroups were comparable across all non-thyroid parameters, including age, BMI, 25(OH)D levels, and lipid fractions. However, free T4 was significantly higher in PTT hyperthyroidism (p < 0.001); free T3 showed a trend toward higher levels in PTT hyperthyroidism (p = 0.052); and TSH was significantly higher in PTT hypothyroidism (p < 0.001). Conclusions: The proportions of patients with PTT hypo- and hyperthyroidism are aligned with international observations. Furthermore, the age was significantly associated with dysthyroidism, and dyslipidemia is the most consistent biochemical correlate of suboptimal thyroid status; however, the associations of PTT dysthyroidism with hypovitaminosis D and BMI were not noticed in this setting.