The Thyroid Gland
Key Points:
Overview
Anatomy and histology of Thyroid Gland
Thyroid hormone synthesis, secretion, peripheral conversion and metabolism
Thyroid hormone receptor
Control of Thyroid Hormone levels
Hypo- and hyperthyroidism
Goitre formation
Biochemical estimates of thyroid function
Thyroid function can be divided into two broad classes: Developmental and Metabolic
Developmental – Essential for normal development, especially in CNS and bone during
early life
Metabolic – Essential for normal metabolism of many body tissues, also in
cardiovascular function
[Congenital Hypothyroidism associated with retarded growth and learning impairment,
linked to iodine deficiency. So congenital hypothyroidism can be easily avoided by
monitoring iodine levels during pregnancy. Thyroid problems in an adult aren’t really
fatal but can make your life miserable]
Insulin is in its own feedback loop, but thyroid hormone is under control of the
hypothalamic-pituitary axis.
Thyroid hormone released from the thyroid gland is stimulated by its stimulating
hormone released from the anterior pituitary gland, this is TSH (thyroid stimulating
hormone/thyrotropin).
Some terms:
Euthyroid = Thyroid function in normal range
Hypothyroid = Thyroid function below normal
Hyperthyroid = Thyroid function is above normal
There are two types of hypo/hyper thyroidism. Primary or secondary.
Primary hypo-/hyperthyroidism is where there is a problem with the thyroid gland
itself
Secondary hypo-/hyperthyroidism is where there is a problem the pituitary regulation
of the thyroid gland
Anatomy of Thyroid Gland
Has two lobes, sometimes a third pyramidal lobe. Also has a very rich blood supply,
more blood per unit weight than the kidney.
Is supplied by inferior thyroid artery from thyrocervical trunk of subclavian artery and
superior thyroid artery as a branch of the external carotid artery.
, The functional unit of thyroid tissue is the follicle.
A follicle consists of a mass of colloid (a protein rich storage material, contains a lot of
hormones available for release). The colloid is surrounded by a single layer of follicular
cells.
It is these follicular cells that synthesise the thyroid hormones and release into colloid
and then take them back up from colloid when there is need for release.
Between the follicles there are capillaries, with the basal membrane of follicular cells
facing the capillary and apical membrane facing the colloid.
Note that there is an additional cell in the thyroid called the C-cell. C-cells secreate
calcitonin. Calcitonin is involved in calcium regulation <- but thought to be minor role.
Thyroid Hormones
Thyroid hormones are derived from (the joining of) two iodinated tyrosine molecules.
There are two thyroid hormones, T4 and T3.
T4 = thyroxine, has 4 iodines
T3 = triiodothyronine, has 3 iodines
Note there is also a reverse T3, which has the iodines in the opposite arrangement. This
is inactive/has no activity.
Importantly, T4 is less active than T3, i.e. T3 is more active at the thyroid receptor.
However T4 is the major form that is released into the blood, so in many ways T4 is like
a prohormone, with possible conversion of T4 -> T3 that can happen in target tissue.
TH Synthesis and Release ->
This will be explained using the diagram below
Key Points:
Overview
Anatomy and histology of Thyroid Gland
Thyroid hormone synthesis, secretion, peripheral conversion and metabolism
Thyroid hormone receptor
Control of Thyroid Hormone levels
Hypo- and hyperthyroidism
Goitre formation
Biochemical estimates of thyroid function
Thyroid function can be divided into two broad classes: Developmental and Metabolic
Developmental – Essential for normal development, especially in CNS and bone during
early life
Metabolic – Essential for normal metabolism of many body tissues, also in
cardiovascular function
[Congenital Hypothyroidism associated with retarded growth and learning impairment,
linked to iodine deficiency. So congenital hypothyroidism can be easily avoided by
monitoring iodine levels during pregnancy. Thyroid problems in an adult aren’t really
fatal but can make your life miserable]
Insulin is in its own feedback loop, but thyroid hormone is under control of the
hypothalamic-pituitary axis.
Thyroid hormone released from the thyroid gland is stimulated by its stimulating
hormone released from the anterior pituitary gland, this is TSH (thyroid stimulating
hormone/thyrotropin).
Some terms:
Euthyroid = Thyroid function in normal range
Hypothyroid = Thyroid function below normal
Hyperthyroid = Thyroid function is above normal
There are two types of hypo/hyper thyroidism. Primary or secondary.
Primary hypo-/hyperthyroidism is where there is a problem with the thyroid gland
itself
Secondary hypo-/hyperthyroidism is where there is a problem the pituitary regulation
of the thyroid gland
Anatomy of Thyroid Gland
Has two lobes, sometimes a third pyramidal lobe. Also has a very rich blood supply,
more blood per unit weight than the kidney.
Is supplied by inferior thyroid artery from thyrocervical trunk of subclavian artery and
superior thyroid artery as a branch of the external carotid artery.
, The functional unit of thyroid tissue is the follicle.
A follicle consists of a mass of colloid (a protein rich storage material, contains a lot of
hormones available for release). The colloid is surrounded by a single layer of follicular
cells.
It is these follicular cells that synthesise the thyroid hormones and release into colloid
and then take them back up from colloid when there is need for release.
Between the follicles there are capillaries, with the basal membrane of follicular cells
facing the capillary and apical membrane facing the colloid.
Note that there is an additional cell in the thyroid called the C-cell. C-cells secreate
calcitonin. Calcitonin is involved in calcium regulation <- but thought to be minor role.
Thyroid Hormones
Thyroid hormones are derived from (the joining of) two iodinated tyrosine molecules.
There are two thyroid hormones, T4 and T3.
T4 = thyroxine, has 4 iodines
T3 = triiodothyronine, has 3 iodines
Note there is also a reverse T3, which has the iodines in the opposite arrangement. This
is inactive/has no activity.
Importantly, T4 is less active than T3, i.e. T3 is more active at the thyroid receptor.
However T4 is the major form that is released into the blood, so in many ways T4 is like
a prohormone, with possible conversion of T4 -> T3 that can happen in target tissue.
TH Synthesis and Release ->
This will be explained using the diagram below
