Iodine Status among Subclinical and Overt Hypothyroid Patients by Urinary Iodine Assay: A Case-Control Study

The objective of the study was to assess the differences of iodine status as measured by urinary iodine excretion (UIE) between cases of hypothyroidism and healthy controls.

The study was conducted in cases with subclinical hypothyroidism (n = 58) and overt hypothyroidism (n = 41) and compared with age- and sex-matched healthy euthyroid controls (n = 52) attending Universal College of Medical Sciences Teaching Hospital, Bhairahawa, Nepal. Serum free triiodothyronine (fT₃), free thyroxine (fT₄), and thyroid-stimulating hormone (TSH) were estimated by competitive ELISA and sandwich ELISA, respectively (Diametra, Italy). The urinary iodine concentration (UIC) in urine samples was estimated by ammonium persulfate digestion method recommended by the WHO.

A significantly higher median UIC was observed among cases of subclinical hypothyroidism (224.90 μg/l) and overt hypothyroidism (281.0 μg/l) as compared to the controls (189.90 μg/l) (P = 0.0001, P = 0.001). Serum TSH in the cases of subclinical hypothyroid was higher, whereas fT₃ was lower as compared to controls (P = 0.028, P = 0.0001), respectively. Similarly, serum TSH in the cases of overt hypothyroid was higher and fT₃ and fT₄ were lower as compared to controls (P = 0.0001, P = 0.0001, P = 0.015), respectively. There was positive correlation of UIC with TSH (r = 0.269, P = 0.0001), whereas negative correlation was seen with fT₃ (r = -0.328, P = 0.0001) and fT₄ (r = -0.145, P = 0.076). The test of multiple regression has shown that fT₃ (β = -0.262, P = 0.012) as an independent predictor in association with UIE in cases.

Excessive iodine intake was found in hypothyroid patients as assessed by UIE concluding that it may trigger the thyroid hypofunction. Cohort studies to generate further evidence should be done to explore potential mechanism of hypothyroidism in excess iodine intake.