OBJECTIVETo explore the effects of thyroid hormone on the expression of homeobox gene Nkx2.1 mRNA in child rat by supplying their hypothyroidism pregnant mother with different dose of levothyroxine (L-thyroxine, L-T(4)) in different times.

METHODS120 female Wistar rats were randomly divided into eight groups according to the body weight: control group, non-treatment hypothyroidism group, hypothyroidism groups supplied with L-T(4) in high, medium and low dosage in early stage (1st-17th day of pregnancy) and in late stage (18th day of pregnancy-20th day after childbirth). According to 100 grams of body weight, the concentrations of L-T(4) were 3.5, 2.0, 0.5 µg/d in high, medium and low dosage group. All the rats were fed with low-iodine food. The control group was given 200 µg/L potassium iodate solution as drinking water and the other groups were given deionized water. After three months, the rats were mated with normal male rats. After the pregnancy was confirmed, hypothyroidism groups were supplied with L-T(4) of different concentrations. Brain samples were taken from the 17-day fetal rats, new-born and 20-day old offsprings and the levels of Nkx2.1 mRNA in brain tissue were analyzed by real-time fluorescence quantitative PCR techniques.

RESULTSThe levels of TT(3) in hypothyroidism groups supplied with L-T(4) in high, medium and low dosages in early and late pregnant stages, non-treatment hypothyroidism group and control group were (0.85 ± 0.17), (0.81 ± 0.18), (0.86 ± 0.21), (0.85 ± 0.20), (0.89 ± 0.18), (0.85 ± 0.20), (0.86 ± 0.20), (1.08 ± 0.07) nmol/L (F = 4.08, P < 0.01); the levels of TT(4) in each group were (0.43 ± 0.16), (0.39 ± 0.11), (0.39 ± 0.13), (0.43 ± 0.17), (0.51 ± 0.19), (0.43 ± 0.16), (0.41 ± 0.15), (39.43 ± 14.16) nmol/L (F = 31.99, P < 0.01); the levels of FT(3) in each group were (3.29 ± 0.61), (3.29 ± 0.61), (3.24 ± 0.61), (3.28 ± 0.63), (3.31 ± 0.59), (3.28 ± 0.50), (3.24 ± 0.49), (4.93 ± 0.46) pmol/L (F = 5.79, P < 0.01); the levels of FT(4) in each group were (3.38 ± 0.80), (3.31 ± 0.67), (3.29 ± 0.73), (3.27 ± 0.71), (3.48 ± 0.81), (3.56 ± 0.66), (3.29 ± 0.61), (27.29 ± 4.53) pmol/L (F = 26.34, P < 0.01). The expression of Nkx2.1 mRNA in non-treatment hypothyroidism group (9.15 × 10(-5) ± 9.17 × 10(-5)) was lower than control group (65.1 × 10(-5) ± 40.90 × 10(-5)) in 17th day of pregnancy (t = 66.224, P < 0.05); the expression of Nkx2.1 mRNA in non-treatment hypothyroidism group (3.16 × 10(-5) ± 0.142 × 10(-5)) was lower than control group (55.6 × 10(-5) ± 51.05 × 10(-5)) in new-born (t = 102.225, P < 0.05); the expression of Nkx2.1 mRNA in non-treatment hypothyroidism group (8.09 × 10(-5) ± 8.21 × 10(-5)) was lower than control group (13.9 × 10(-5) ± 7.43 × 10(-5)) in 20th day after birth (t = 9.235, P < 0.05). The trend of Nkx2.1 mRNA in hypothyroidism groups was decreased in group supplied with L-T(4) in medium dosage in early stage descends in 17th day of pregnancy, new-born and 20th day after birth (57.1 × 10(-5) ± 22.90 × 10(-5)), (30.8 × 10(-5) ± 27.20 × 10(-5)), (17.1 × 10(-5) ± 0.623 × 10(-5)) (F = 13.394, P < 0.01). The expression of Nkx2.1 mRNA in hypothyroidism groups supplied with L-T(4) in medium dosage in early stage in 17th day of pregnancy, new-born and 20th day after childbirth was closest to the control group in every period (t values were 0.225, 0.336, 0.345, all P values > 0.05).

CONCLUSIONThe difference in the expression of homeobox gene Nkx2.1 mRNA is highly related to the level of thyroid hormone.