254
AUBREY CORBMAN
man (1962) and others have pointed out, the rates of uptake and discharge ( = turnover) of iodine by the thyroid is subject not only to intrinsic properties of the fish thyroid (including regulatory influences from
the hypothalamo-hypophysial system) but also to other factors like environmental salinity, temperature, and iodine concentration. Fishes differ
also in the proportion of thyroxine to triiodothyronine formed in the
thyroid and released into the blood. An extreme instance is the mud minnow, Umbra limi, which under certain natural circumstances makes considerable amounts of triiodothyronine but no thyroxine. Ordinarily, however, triiodothyronine is relatively difficult to demonstrate on the fish
thyroid (Berg et al., 1959 ) .
The goldfish, Carassius auratus, and “Pumpkinseed,” Lepomis gibbow, have been found by Berg and Gorbman ( 1954), Berg et al. ( 1959),
and Fortune (1956) to represent those fish with very “slow” thyroids.
That is, they accumulate in the pharyngeal thyroid only a small fraction
of an administered dose of radioiodine and convert this to thyroxine to a
very small degree, and only after a period of more than a week. Part of
the explanation of the low pharyngeal iodine uptake is provided by Chavin
(1956), and Chavin and Bouwman (1965) who show that the thyroid
tissue in the head kidney accumulates about twice as much radioiodine as
does the pharyngeal thyroid. Goldfish head kidney thyroid is more sensitive to the radioactivity of radioiodine than is pharyngeal thyroid (Chavin
and Bouwman, 1965). This fits well with the observation of Baker-Cohen
(1959) that in platyfish whose thyroid tissue is hyperplastic because of a
low iodine environment, the pattern of hyperplasia is quite frequently
different between pharynx and head kidney or splenic thyroid. A puzzling
datum in the publication of Chavin and Bouwman (1965) is the finding
of a maximum thyroidal uptake at 60 min after injection of radioiodine.
This differs from the experience of others and raises the possibility that
variations in experimental design or conditions for maintaining teleost
fish may be the source of important discrepancies in work from different
laboratories. Another remarkable datum from Chavin and Bouwman
(1965) is the claim that in goldfish kidney thyroid, which has a maximum
uptake of 7% of the injected 1311 at 60 min and a fall from this maximum
to 1.7% at 2.4 hr, the lS1I “uptake” rises again to 4.4% at 84 days. This would
imply an extraordinary retention of iodine by the goldfish and a slow
recycling to the thyroid after long periods in the tissues.
In salmonids special interest has been attached to correlations of
thyroid function with phases of the life cycle. Hoffert and Fromm (1959)
and Hunn and Fromm (1964) for example, estimated thyroxine secretion
rate in trout and found a 5-10 times greater rate between the immature
young and the 1- to 2-year-old. Jacoby and Hickman (1966), using an
AUBREY CORBMAN
man (1962) and others have pointed out, the rates of uptake and discharge ( = turnover) of iodine by the thyroid is subject not only to intrinsic properties of the fish thyroid (including regulatory influences from
the hypothalamo-hypophysial system) but also to other factors like environmental salinity, temperature, and iodine concentration. Fishes differ
also in the proportion of thyroxine to triiodothyronine formed in the
thyroid and released into the blood. An extreme instance is the mud minnow, Umbra limi, which under certain natural circumstances makes considerable amounts of triiodothyronine but no thyroxine. Ordinarily, however, triiodothyronine is relatively difficult to demonstrate on the fish
thyroid (Berg et al., 1959 ) .
The goldfish, Carassius auratus, and “Pumpkinseed,” Lepomis gibbow, have been found by Berg and Gorbman ( 1954), Berg et al. ( 1959),
and Fortune (1956) to represent those fish with very “slow” thyroids.
That is, they accumulate in the pharyngeal thyroid only a small fraction
of an administered dose of radioiodine and convert this to thyroxine to a
very small degree, and only after a period of more than a week. Part of
the explanation of the low pharyngeal iodine uptake is provided by Chavin
(1956), and Chavin and Bouwman (1965) who show that the thyroid
tissue in the head kidney accumulates about twice as much radioiodine as
does the pharyngeal thyroid. Goldfish head kidney thyroid is more sensitive to the radioactivity of radioiodine than is pharyngeal thyroid (Chavin
and Bouwman, 1965). This fits well with the observation of Baker-Cohen
(1959) that in platyfish whose thyroid tissue is hyperplastic because of a
low iodine environment, the pattern of hyperplasia is quite frequently
different between pharynx and head kidney or splenic thyroid. A puzzling
datum in the publication of Chavin and Bouwman (1965) is the finding
of a maximum thyroidal uptake at 60 min after injection of radioiodine.
This differs from the experience of others and raises the possibility that
variations in experimental design or conditions for maintaining teleost
fish may be the source of important discrepancies in work from different
laboratories. Another remarkable datum from Chavin and Bouwman
(1965) is the claim that in goldfish kidney thyroid, which has a maximum
uptake of 7% of the injected 1311 at 60 min and a fall from this maximum
to 1.7% at 2.4 hr, the lS1I “uptake” rises again to 4.4% at 84 days. This would
imply an extraordinary retention of iodine by the goldfish and a slow
recycling to the thyroid after long periods in the tissues.
In salmonids special interest has been attached to correlations of
thyroid function with phases of the life cycle. Hoffert and Fromm (1959)
and Hunn and Fromm (1964) for example, estimated thyroxine secretion
rate in trout and found a 5-10 times greater rate between the immature
young and the 1- to 2-year-old. Jacoby and Hickman (1966), using an