4. THYROID FUNCTION AND ITS CONTROL IN FISHES
255
isotope dilution method, were able to estimate absolute concentration of
iodocompounds in adult rainbow trout. They found totals of 1.1 pg of
thyroxine, 1.3 pg of triiodothyronine, and 1.1 fig of iodotyrosines per 100
ml of blood. It would be interesting to reconcile these data with Hoffert
and Fromm’s to note the relation between thyroid secretion rate and
blood levels of hormone. The presence of so large a proportion of iodotyrosine in the blood is surprising, and it needs further study.
Another remarkable claim that requires additional study and confumation is made by La Roche et al. (1966a), who have attempted radiothyroidectomy of trout and of chinook salmon by repeated injection of
1311. While thyroidectomized trout underwent characteristic thyroidectomy changes, the salmon did not. Nine months after destruction of thyroid follicles, radiothyroxine still was produced by the “radiothyroidectomized” salmon from a tracer 1311 dose. The explanation offered by La
Roche et a2. is that disorganized nonfollicular thyroid cells may be performing this function. An equally good possibility is the survival of
follicular tissue in one of the many sites mentioned by Baker-Cohen
(1959) where heterotopic thyroid could be found in platyfish. The intriguing questions raised by La Roche et al. require further study.
The influences of fluctuations of environmental factors, and of TSH,
on thyroid metabolism have been studied in fragmentary fashion by many
authors (Leloup and Fontaine, 1960; Berg and Gorbman, 1954; Gorbman
and Berg, 1955; Hickman, 1959; Hunn and Reineke, 1964). However,
probably no study of biochemical changes in iodine metabolism has been
made in as extensive a manner in fishes as that by Hickman (1962). The
particular value of this study is that it provides data on stable iodine
( I Z T I ) changes as well as the shifts of tracer 1311. As Fig. 4 indicates, there
is a complex of changes in iodide content, PBI (protein bound iodine, considered all organified iodine), and BE1 (butanol extractable iodine, considered to be iodine in hormonal form) over a period as long as 1&12 days
after a single injection of TSH into a flounder, Plutichthys stellatus. The
effect of repeated injections of TSH on the same species is shown in
Fig. 5. It is most interesting that in Platichthys there is a remarkable loss
of stable (total) iodine at a time when the 48-hr radioiodine uptake is
greatly increased in the thyroid. In the blood of Platichthys the most sensitive indicator of TSH action appears to be the ratio of PBI to iodide, a
fact that underlines the usefulness of “conversion ratio” in the blood
(PB1311/total 1 3 1 1 ) to easily indicate thyroid state in fish. Use of conversion ratio (CR) for this purpose has been recommended by Hickman
(1961) and by Eales ( 1963). The extension of these principles to the
rainbow trout has been made also by Hickman (1962) where the oppositeness in response of stable iodine content and lS1I uptake in thyroid
255
isotope dilution method, were able to estimate absolute concentration of
iodocompounds in adult rainbow trout. They found totals of 1.1 pg of
thyroxine, 1.3 pg of triiodothyronine, and 1.1 fig of iodotyrosines per 100
ml of blood. It would be interesting to reconcile these data with Hoffert
and Fromm’s to note the relation between thyroid secretion rate and
blood levels of hormone. The presence of so large a proportion of iodotyrosine in the blood is surprising, and it needs further study.
Another remarkable claim that requires additional study and confumation is made by La Roche et al. (1966a), who have attempted radiothyroidectomy of trout and of chinook salmon by repeated injection of
1311. While thyroidectomized trout underwent characteristic thyroidectomy changes, the salmon did not. Nine months after destruction of thyroid follicles, radiothyroxine still was produced by the “radiothyroidectomized” salmon from a tracer 1311 dose. The explanation offered by La
Roche et a2. is that disorganized nonfollicular thyroid cells may be performing this function. An equally good possibility is the survival of
follicular tissue in one of the many sites mentioned by Baker-Cohen
(1959) where heterotopic thyroid could be found in platyfish. The intriguing questions raised by La Roche et al. require further study.
The influences of fluctuations of environmental factors, and of TSH,
on thyroid metabolism have been studied in fragmentary fashion by many
authors (Leloup and Fontaine, 1960; Berg and Gorbman, 1954; Gorbman
and Berg, 1955; Hickman, 1959; Hunn and Reineke, 1964). However,
probably no study of biochemical changes in iodine metabolism has been
made in as extensive a manner in fishes as that by Hickman (1962). The
particular value of this study is that it provides data on stable iodine
( I Z T I ) changes as well as the shifts of tracer 1311. As Fig. 4 indicates, there
is a complex of changes in iodide content, PBI (protein bound iodine, considered all organified iodine), and BE1 (butanol extractable iodine, considered to be iodine in hormonal form) over a period as long as 1&12 days
after a single injection of TSH into a flounder, Plutichthys stellatus. The
effect of repeated injections of TSH on the same species is shown in
Fig. 5. It is most interesting that in Platichthys there is a remarkable loss
of stable (total) iodine at a time when the 48-hr radioiodine uptake is
greatly increased in the thyroid. In the blood of Platichthys the most sensitive indicator of TSH action appears to be the ratio of PBI to iodide, a
fact that underlines the usefulness of “conversion ratio” in the blood
(PB1311/total 1 3 1 1 ) to easily indicate thyroid state in fish. Use of conversion ratio (CR) for this purpose has been recommended by Hickman
(1961) and by Eales ( 1963). The extension of these principles to the
rainbow trout has been made also by Hickman (1962) where the oppositeness in response of stable iodine content and lS1I uptake in thyroid