xine, with the expulsion of an alanine side chain. Triiodorhryonine may be formed
by the removal of one of the iodine atoms from thyroxine or, as considered most
likely, by the separate condensation of mono-and di-iodotyrosines. The hormones
are stored in combination with a large glycoprotein molecule, thyroglobulin, which
make up the colloid in the follicles, and are released from this by the action of
a protease.
The activity of the thyroid gland may be judged in several ways . The histological
appearance varies, the secretory cells being tall and columnar when it is in an active
state, and low and squamous when inactive . The quantity of protein-bound iodine
and the uptake of radioiodine (
1311)
is greatest in active glands . The significance of
such changes, however, mu st be interpreted with considerable caution, as the y do
not necessarily reflect circulating blood levels of the hormones. The environmental
iodine levels thus may profoundly affect such values; limited availability will result
in the appearance of a highly active gland, yet the animal may be functionally
hypothyroid. The pharmacological standardization of therapeutically used thyroid
gland preparations can be carried out by measurement of the protein-bound iodine
in the gland. A batch of such material once released on the American market was
found to be almost devoid of biological activity, but it is not clear whether this
was due to a biological inconsistency, or a human one of 'salting' the material with
iodine.
b) Function
The uniform presence of thyroid tissues and secretions throughout the vertebrates,
along with the persistence of the controlling pituitary thyrotrophin-thyroid axis,
strongly suggests that it has a ph ysiological role in all vertebrates. This has been -
well characterized in the mammals, birds and larval amphibians, but despite extensive investigations an unequivocal function has not been established in the rest of
the vertebrates.
In the homoiotherrns the thyroid hormones have a calorigenic function and
stimulate metabolism and oxygen consumption. In the poikilotherms, a similar role
has been claimed in some species under certain conditions, but most evidence
strongly indicates that this is not a characteristic action in such animals.
The thyroid is also necessary for optimal growth and differentiation in some
species, but again, this has only been consistently shown in the hornoiotherms.
Similar claims have been made among the poikilotherms, especially the fishes, but
a consensus has not been attained. The thyroid secretions are necessary for the metamorphic transformation of larval Amphibia to the adult form, the morphogenetic
effect, but while evidence for a comparable role in fishes has been sought, it has
not been forthcoming.
Evidence derived principally from homoiotherms, suggests that the thyroid secretions have a diffuse action on all the cells in the body and, by a process which
is not yet clear, facilitate their optimal function, growth and differentiation. In the
poikilotherms changes of thyroid activity have been associated with alterations in
their physiological and environmental situation including the reproductive cycle,
migration, temperature, life in salt and fresh water and movements between such
61
by the removal of one of the iodine atoms from thyroxine or, as considered most
likely, by the separate condensation of mono-and di-iodotyrosines. The hormones
are stored in combination with a large glycoprotein molecule, thyroglobulin, which
make up the colloid in the follicles, and are released from this by the action of
a protease.
The activity of the thyroid gland may be judged in several ways . The histological
appearance varies, the secretory cells being tall and columnar when it is in an active
state, and low and squamous when inactive . The quantity of protein-bound iodine
and the uptake of radioiodine (
1311)
is greatest in active glands . The significance of
such changes, however, mu st be interpreted with considerable caution, as the y do
not necessarily reflect circulating blood levels of the hormones. The environmental
iodine levels thus may profoundly affect such values; limited availability will result
in the appearance of a highly active gland, yet the animal may be functionally
hypothyroid. The pharmacological standardization of therapeutically used thyroid
gland preparations can be carried out by measurement of the protein-bound iodine
in the gland. A batch of such material once released on the American market was
found to be almost devoid of biological activity, but it is not clear whether this
was due to a biological inconsistency, or a human one of 'salting' the material with
iodine.
b) Function
The uniform presence of thyroid tissues and secretions throughout the vertebrates,
along with the persistence of the controlling pituitary thyrotrophin-thyroid axis,
strongly suggests that it has a ph ysiological role in all vertebrates. This has been -
well characterized in the mammals, birds and larval amphibians, but despite extensive investigations an unequivocal function has not been established in the rest of
the vertebrates.
In the homoiotherrns the thyroid hormones have a calorigenic function and
stimulate metabolism and oxygen consumption. In the poikilotherms, a similar role
has been claimed in some species under certain conditions, but most evidence
strongly indicates that this is not a characteristic action in such animals.
The thyroid is also necessary for optimal growth and differentiation in some
species, but again, this has only been consistently shown in the hornoiotherms.
Similar claims have been made among the poikilotherms, especially the fishes, but
a consensus has not been attained. The thyroid secretions are necessary for the metamorphic transformation of larval Amphibia to the adult form, the morphogenetic
effect, but while evidence for a comparable role in fishes has been sought, it has
not been forthcoming.
Evidence derived principally from homoiotherms, suggests that the thyroid secretions have a diffuse action on all the cells in the body and, by a process which
is not yet clear, facilitate their optimal function, growth and differentiation. In the
poikilotherms changes of thyroid activity have been associated with alterations in
their physiological and environmental situation including the reproductive cycle,
migration, temperature, life in salt and fresh water and movements between such
61