4
THYROID FUNCTION AND
ITS CONTROL IN FISHES
AUBREY GORBMAN
I. Introduction . . . . . . . . . . . . 241
11. Hypothalamic and Other Regulation of TSH Secretion . . . 242
111. Thyrotropic Function of the Fish Pituitary Gland . . . . 245
IV. Thyroid Function and Thyroid Hormone Synthesis . . . . 249
V. Actions of Thyroid Hormones in Fish
. . . . . . 260
References . . . . . . . . . . . . . 265
I. INTRODUCTION
It has not been possible to make this chapter an exhaustive review of
thyroidal function in fishes, nor is such a review really desirable. Much of
the research on piscine thyroid physiology has merely established that in
the broad sense it is not too different from thyroid function in other vertebrates. What makes the study of controlled thyroid function in fish exciting to the comparative endocrinologist is that some differences do exist.
Furthermore, since these characteristic piscine differences are found in
the most primitive of living vertebrates, they offer the only clues available
concerning the evolution of thyroid function.
The general vertebrate pattern of thyroid function is diagrammed in
Fig. 1. The four components of this functional system are the following:
(1) The brain, and more precisely a complex of nervous af€erents to a
center (or centers ), presumably in the hypothalamus, which regulate
pituitary thyrotropic (TSH) secretion. This part of the system links
changes in the extrinsic environment ( e.g., temperature and photoperiod)
to eventual changes in thyroid secretion rate. The brain may contain
thyroxine-sensitive centers, and possibly these are the same as the hypothalamic centers just referred to; they are part of a feedback mechanism
241
THYROID FUNCTION AND
ITS CONTROL IN FISHES
AUBREY GORBMAN
I. Introduction . . . . . . . . . . . . 241
11. Hypothalamic and Other Regulation of TSH Secretion . . . 242
111. Thyrotropic Function of the Fish Pituitary Gland . . . . 245
IV. Thyroid Function and Thyroid Hormone Synthesis . . . . 249
V. Actions of Thyroid Hormones in Fish
. . . . . . 260
References . . . . . . . . . . . . . 265
I. INTRODUCTION
It has not been possible to make this chapter an exhaustive review of
thyroidal function in fishes, nor is such a review really desirable. Much of
the research on piscine thyroid physiology has merely established that in
the broad sense it is not too different from thyroid function in other vertebrates. What makes the study of controlled thyroid function in fish exciting to the comparative endocrinologist is that some differences do exist.
Furthermore, since these characteristic piscine differences are found in
the most primitive of living vertebrates, they offer the only clues available
concerning the evolution of thyroid function.
The general vertebrate pattern of thyroid function is diagrammed in
Fig. 1. The four components of this functional system are the following:
(1) The brain, and more precisely a complex of nervous af€erents to a
center (or centers ), presumably in the hypothalamus, which regulate
pituitary thyrotropic (TSH) secretion. This part of the system links
changes in the extrinsic environment ( e.g., temperature and photoperiod)
to eventual changes in thyroid secretion rate. The brain may contain
thyroxine-sensitive centers, and possibly these are the same as the hypothalamic centers just referred to; they are part of a feedback mechanism
241