4. THYROID FUNCTION AND ITS CONTROL IN FISHES
W9
indications of this tabulation is that the criteria for jjdging thyroid function with respect to temperature may vary in an opposite way, contributing to some of the confusion that this topic enjoys. As Drury and Eales
(1968) point out, thyroid activity judged by histology may be activated,
depressed, or remain unchanged with respect to temperature variation,
but radioiodine metabolic measures generally change in a direction
parallel with temperature. There are two apparent exceptions, however,
even to this generalization: the mud minnow and the brown trout, It
would seem that the dissociation of the histological response and the
iodine metabolic response of teleost thyroids when the temperature is
changed means that they are under separate and different control. A part
of the reaction may be in response to a temperature actuated hypothalamohypophysial-TSH mechanism, and the rest to a direct effect of temperature on the thyroid. Further work, particularly with hypophysectomized
fish, may help to clarify this problem. Leloup and Fontaine (1960) have
reported that in hypophysectomized eels radioiodine levels are similar and
low at both 6.5" and 25°C. At 6.5"C TSH has little or no effect upon thyroidal radioiodine uptake either in hypophysectomized or normal eels.
These data indicate that part of the temperature effect may be explained
on the basis of difference of kinetics of action of TSH at higher versus
lower temperatures.
There is some special interest in the function of the lungfish thyroid
in the humid and in the dry or cocoon state. This interest derives from
the fact that an environmental factor other than temperature or light
appears to be regulating thyroid function in this species, and it may be
a derivative of the state of hydration of the animal. Both Godet and Dup6
( 1962) and Leloup (1958b, 1963), as well as Leloup and Fontaine
( 1980), have described the changes in thyroidal l3II uptake in Protopterus
annectens in the wet and dry states, but kept at the same temperature.
Both groups find that passage from the wet to the dry phase causes a
sharp decrease in thyroid radioiodine uptake. Leloup, in addition, has
shown that this is accompanied by a fall in plasma organic iodine and a
lowered secretion rate by the thyroid. Returning a dry lungfish to a wet
environment causes a prompt activation of the thyroid, revealed by a
prompt loss of previously stored thyroidal radioiodine.
Analysis of the thyroid iodoproteins by density gradient centrifugation
has been done by Lachiver et a2. (1965). Three fractions were obtained
from eel thyroid (11.1 S, 17.8 S, 25.4 S). Of these the 11.1 S protein is
very feebly iodinated, but the others are highly iodinated. In trout, similarly, the principal thyroglobulin fraction (about 16 S) is iodinated. In
moist or dry lungfish, Protopterus annectens, most radioiodine activity is
associated with a fraction whose sedimentation rate is 20.8 S, but radio-
W9
indications of this tabulation is that the criteria for jjdging thyroid function with respect to temperature may vary in an opposite way, contributing to some of the confusion that this topic enjoys. As Drury and Eales
(1968) point out, thyroid activity judged by histology may be activated,
depressed, or remain unchanged with respect to temperature variation,
but radioiodine metabolic measures generally change in a direction
parallel with temperature. There are two apparent exceptions, however,
even to this generalization: the mud minnow and the brown trout, It
would seem that the dissociation of the histological response and the
iodine metabolic response of teleost thyroids when the temperature is
changed means that they are under separate and different control. A part
of the reaction may be in response to a temperature actuated hypothalamohypophysial-TSH mechanism, and the rest to a direct effect of temperature on the thyroid. Further work, particularly with hypophysectomized
fish, may help to clarify this problem. Leloup and Fontaine (1960) have
reported that in hypophysectomized eels radioiodine levels are similar and
low at both 6.5" and 25°C. At 6.5"C TSH has little or no effect upon thyroidal radioiodine uptake either in hypophysectomized or normal eels.
These data indicate that part of the temperature effect may be explained
on the basis of difference of kinetics of action of TSH at higher versus
lower temperatures.
There is some special interest in the function of the lungfish thyroid
in the humid and in the dry or cocoon state. This interest derives from
the fact that an environmental factor other than temperature or light
appears to be regulating thyroid function in this species, and it may be
a derivative of the state of hydration of the animal. Both Godet and Dup6
( 1962) and Leloup (1958b, 1963), as well as Leloup and Fontaine
( 1980), have described the changes in thyroidal l3II uptake in Protopterus
annectens in the wet and dry states, but kept at the same temperature.
Both groups find that passage from the wet to the dry phase causes a
sharp decrease in thyroid radioiodine uptake. Leloup, in addition, has
shown that this is accompanied by a fall in plasma organic iodine and a
lowered secretion rate by the thyroid. Returning a dry lungfish to a wet
environment causes a prompt activation of the thyroid, revealed by a
prompt loss of previously stored thyroidal radioiodine.
Analysis of the thyroid iodoproteins by density gradient centrifugation
has been done by Lachiver et a2. (1965). Three fractions were obtained
from eel thyroid (11.1 S, 17.8 S, 25.4 S). Of these the 11.1 S protein is
very feebly iodinated, but the others are highly iodinated. In trout, similarly, the principal thyroglobulin fraction (about 16 S) is iodinated. In
moist or dry lungfish, Protopterus annectens, most radioiodine activity is
associated with a fraction whose sedimentation rate is 20.8 S, but radio-