Toxicology of Industrial Compounds

Figure 19.3 Thyroid follicular epithelial cell function.
C.K.ATTERWILL AND S.P.AYLWARD 259
Control of thyroid function and pathobiology of thyroid
lesions
Xenobiotic toxic effects on the hypothalamic-pituitarythyroid-liver
(H-P-T-L) axis
The control of mammalian thyroid follicular function is shown
schematically in Figure 19.2 together with the points at which agents may
perturb function and cause toxicity and thyroid lesions. The most common
classes of agent affecting function are discussed at the different loci in
terms of the categories (1–4) of endocrine toxicity (Atterwill and Flack,
1993).
The various control mechanisms and factors influencing hormone
synthesis, distribution and metabolism can be summarised as follows:
thyroid hormone (T 3 and T 4) synthesis and secretion from thyroid gland
are controlled by thyroid stimulating hormone (TSH) released from the
pituitary gland. This in turn is under control by hypothalamic thyrotrophin
releasing hormone (TRH) and circulating levels of the thyroid hormones.
Thyroid hormones exist in the circulation in the free (free T 4 (FT 4) and
free T 3 (FT 3)) and protein-bound forms (approx 99 per cent of total T 4-
TT 4 and total T 3-TT 3) and it is the FT 3 hormone produced by deiodination
from T 4 which has both a physiological action on T 3 nuclear receptors in
target tissues and influences pituitary TSH output.