osmotic media. Thus, it is often felt that th yroid hormones have a widespread role
in th e life functions of such animals. Nevertheless, fish may get along perfectly
well after thei r th yroid has been removed (MAlTY, 1957).
c) Relationship to Osmoregulation
While there is a lack of evidence concerning a direct role of the th yroid hormones
in osmoregulation, there are man y examples of their ph ysiological interactions. The
osmotic situation of an animal may affect th yroid function by influencing the
amounts of iodide that are available; less iodine is available to fish in fresh water
than to those living in the sea (GORBMAN, 1963). Goitrogenic sub stances that inhibit
the formation of the thyroid hormones occur in nature, and these include excesses
of sodium chloride. Mice given sodium chloride solutions to ingest suffer from
hypothyroidism, as renal excretion of the excess salt also results in loss of iodide
(ISLER, LEBLOND, and AXELRAD, 1958). Thus care must be taken to distinguish between those effects of environmental osmotic changes on thyroid activity that are
due to altered iodine metabolism, and a supposed additional osmoregulatory requirement for the hormone.
The calorigenic effects of th yroid hormones in homoiotherms will increase the
respiratory water loss that accompanies the increased respiratory gas exchange.
Evaporative water losses from the skin may also be increased , especially if there is
a resulting need for thermoregulation.
Inhibition of th yroid function in rats results in an increased exchange of sodium
chloride and water. This is due to a failure of the kidney to conserve salt adequately; the renal tubules do not reabsorb as much sodium, their sensitivity to aldosterone is reduced about ten-fold, and the production of this steroid hormone
is depressed (FREGLY and TAYLOR, 1964). The effects of th e antidiuretic hormone
are also reduced. In ducks, th yroidectomy results in a dela y in th~ onset of secretion
from the nasal salt gland following the administration of sodium chloride solutions
(ENSOR, THOMAS, and PHILLIPS, 1970). This probably reflects a dela y in the absorption of the saline from the gut . It is not known whether such actions extend
to poikilotherms, but it has been suggested that seasonal changes in the sensitivity
of frogs to neurohypophysial hormones, ma y be mediated by the th yroid gland
(HELLER, 1930).
The 'maturation effect' of thyroid hormones in the Amphibia profoundly affects
their water and salt metabolism. The metamorphosed adult animals usually assume
a terrestrial, as opposed to an aquatic manner of life, and this is associated with
the development of a number of relevant ph ysiological processes. Tadpoles of the
bullfrog, Rana catesbeiana, do not appear to transport sodium actively across their
skin until after the fore and hind limbs appear, a process that can be accelerated
by exposure to th yroxine (TAYLOR and BARKER, 1965). Amphibian tadpoles also
respond poorly, if at all, to the neurohypophysial hormones, but th e response is
facilitated after metamorphosis (HOWES, 1940).
The migration of fishes is accompanied by increased activity of the th yroid
gland (BARRINGTON, 1963). Such migration often takes the animals into a contrasting osmotic milieu, such as fresh water from th e sea, or in the opposite
62
in th e life functions of such animals. Nevertheless, fish may get along perfectly
well after thei r th yroid has been removed (MAlTY, 1957).
c) Relationship to Osmoregulation
While there is a lack of evidence concerning a direct role of the th yroid hormones
in osmoregulation, there are man y examples of their ph ysiological interactions. The
osmotic situation of an animal may affect th yroid function by influencing the
amounts of iodide that are available; less iodine is available to fish in fresh water
than to those living in the sea (GORBMAN, 1963). Goitrogenic sub stances that inhibit
the formation of the thyroid hormones occur in nature, and these include excesses
of sodium chloride. Mice given sodium chloride solutions to ingest suffer from
hypothyroidism, as renal excretion of the excess salt also results in loss of iodide
(ISLER, LEBLOND, and AXELRAD, 1958). Thus care must be taken to distinguish between those effects of environmental osmotic changes on thyroid activity that are
due to altered iodine metabolism, and a supposed additional osmoregulatory requirement for the hormone.
The calorigenic effects of th yroid hormones in homoiotherms will increase the
respiratory water loss that accompanies the increased respiratory gas exchange.
Evaporative water losses from the skin may also be increased , especially if there is
a resulting need for thermoregulation.
Inhibition of th yroid function in rats results in an increased exchange of sodium
chloride and water. This is due to a failure of the kidney to conserve salt adequately; the renal tubules do not reabsorb as much sodium, their sensitivity to aldosterone is reduced about ten-fold, and the production of this steroid hormone
is depressed (FREGLY and TAYLOR, 1964). The effects of th e antidiuretic hormone
are also reduced. In ducks, th yroidectomy results in a dela y in th~ onset of secretion
from the nasal salt gland following the administration of sodium chloride solutions
(ENSOR, THOMAS, and PHILLIPS, 1970). This probably reflects a dela y in the absorption of the saline from the gut . It is not known whether such actions extend
to poikilotherms, but it has been suggested that seasonal changes in the sensitivity
of frogs to neurohypophysial hormones, ma y be mediated by the th yroid gland
(HELLER, 1930).
The 'maturation effect' of thyroid hormones in the Amphibia profoundly affects
their water and salt metabolism. The metamorphosed adult animals usually assume
a terrestrial, as opposed to an aquatic manner of life, and this is associated with
the development of a number of relevant ph ysiological processes. Tadpoles of the
bullfrog, Rana catesbeiana, do not appear to transport sodium actively across their
skin until after the fore and hind limbs appear, a process that can be accelerated
by exposure to th yroxine (TAYLOR and BARKER, 1965). Amphibian tadpoles also
respond poorly, if at all, to the neurohypophysial hormones, but th e response is
facilitated after metamorphosis (HOWES, 1940).
The migration of fishes is accompanied by increased activity of the th yroid
gland (BARRINGTON, 1963). Such migration often takes the animals into a contrasting osmotic milieu, such as fresh water from th e sea, or in the opposite
62