Whole-body content of (A) thyroxine (T₄); (B) free thyroxine (FT₄); (C) triiodothyronine (T₃); and (D) free triiodothyronine (FT₃) in juvenile zebrafish after exposure to different concentrations of microcystin-LR (MC-LR) (0, 1, 5, and 25 μg/L) for 7, 14, 21, and 28 days. The values are expressed as mean ± SD (n = 3). Significant differences obtained by one-way ANOVA followed by least significant difference (LSD) test are indicated between control and exposed groups, *p < 0.05, **p < 0.01.

Whole-body content of (A) thyroxine (T₄); (B) free thyroxine (FT₄); (C) triiodothyronine (T₃); and (D) free triiodothyronine (FT₃) in juvenile zebrafish after exposure to different concentrations of microcystin-LR (MC-LR) (0, 1, 5, and 25 μg/L) for 7, 14, 21, and 28 days. The values are expressed as mean ± SD (n = 3). Significant differences obtained by one-way ANOVA followed by least significant difference (LSD) test are indicated between control and exposed groups, *p < 0.05, **p < 0.01.

The effects of exposure to different concentrations of microcystin-LR (MC-LR) for 28 days on the histological structure of thyroid follicles in juvenile zebrafish; (A) Control; (B) 1 μg/L MC-LR; (C) 5 μg/L MC-LR; and (D) 25 μg/L MC-LR, (C in figures = colloid; single arrows = hypertrophy; double arrows = hyperplasia).

The effects of exposure to different concentrations of microcystin-LR (MC-LR) for 28 days on the histological structure of thyroid follicles in juvenile zebrafish; (A) Control; (B) 1 μg/L MC-LR; (C) 5 μg/L MC-LR; and (D) 25 μg/L MC-LR, (C in figures = colloid; single arrows = hypertrophy; double arrows = hyperplasia).

Thyroid follicle cell nuclear size following exposure of juvenile zebrafish to microcystin-LR (MC-LR) for 28 days (n = 3). Long and short nuclear diameters were used to estimate the cross-sectional area of at least 35 follicle cell nuclei per fish. Significant differences obtained by one-way ANOVA followed by least significant difference (LSD) test are indicated between control and exposed groups, **p < 0.01.

Percent of hyperplasia follicles in juvenile zebrafish exposed to microcystin-LR (MC-LR) for 28 days (n = 3). Significant differences obtained by one-way ANOVA followed by least significant difference (LSD) test are indicated between control and exposed groups, *p < 0.05, **p < 0.01.

Transcript abundance for (A) corticotropin-releasing hormone (CRH); (B) thyroid-stimulating hormone (TSH); (C) thyroid peroxidase (TPO); and (D) transthyretin (TTR) in juvenile zebrafish after exposure to different concentrations of microcystin-LR (MC-LR) (0, 1, 5, and 25 μg/L) for 7, 14, 21, and 28 days. The values are expressed as mean ± SD (n = 3). Significant differences obtained by one-way ANOVA followed by least significant difference (LSD) test are indicated between control and exposed groups, *p < 0.05, **p < 0.01.

The activity of (A) type I iodothyronine deiodinase (ID1); (B) type II iodothyronine deiodinase (ID2); and (C) type III iodothyronine deiodinase (ID3) in juvenile zebrafish after exposure to different concentrations of microcystin-LR (MC-LR) (0, 1, 5, and 25 μg/L) for 7, 14, 21, and 28 days. The values are expressed as mean ± SD (n = 3). Significant differences obtained by one-way ANOVA followed by least significant difference (LSD) test are indicated between control and exposed groups, *p < 0.05, **p < 0.01.