Hypothyroidism, or underactivity of the thyroid gland, results from failure
to maintain adequate levels of thyroid hormones. It is caused by any
structural or functional derangement that interferes with the thyroid glands
ability to produce adequate hormone. Because this can occur anywhere along
the hypothalamic-pituitary-thyroid axis, hypothyroidism can be divided into
primary, secondary, and tertiary subcategories. The two most
common causes are autoimmune thyroiditis (Hashimoto's thyroiditis) and previous
treatment for hyperthyroidism (total or subtotal thyroidectomy or radioablation
In primary hypothyroidism, there is an intrinsic inability of the thyroid
gland to produce hormone. Common causes of primary hypothyroidism include
functional problems within the thyroid gland, infiltrative disease of the
thyroid, silent or subacute thyroiditis, chronic autoimmune thyroiditis,
radioiodine therapy, and postoperative hypothyroidism. External radiation
to the neck (particularly in children) has also been known to cause
hypothyroidism. In addition, anything which interrupts thyroid hormone
synthesis or causes defects in thyroglobulin can result in hypothyroidism, such
as iodine deficiency, antithyroid medications, congenital defects in synthesis,
In secondary and tertiary hypothyroidism, the thyroid gland is intrinsically
normal, and the problem lies within the pituitary or hypothalamus. Here,
the thyroid gland functions normally, however, it does not receive stimulation
from the pituitary or brain to create thyroid hormones. In secondary
hypothyroidism, problems with the pituitary cause decreased synthesis and
release of TSH. In tertiary hypothyroidism, problems within the
hypothalamus cause decreased synthesis of TRH and subsequent decreased
stimulation of the pituitary.
Hypothyroidism typically manifests as a slowing in physical and mental
activity. The signs and symptoms can be quite subtle and typically do not
present themselves unless a careful search is made for them, and in fact, are
often dismissed as the "normal part of aging".
Symptoms include weight gain, cold intolerance, fatigue, impaired cognitive
function, dull facial expression, coarse facial features, dry and brittle hair,
slow movements and speech, pallor, bradycardia, goiter, and constipation to name
a few. Symptoms arise in response to the decreased levels of serum thyroid
hormones and hypometabolic state.
Lab studies play a role in the diagnosis of hypothyroidism. A TSH assay
is the most useful test in screening for primary hypothyroidism and for
confirming the diagnosis. TSH is increased in hypothyroidism because there
are no or decreased hormone levels to negatively feed back on the pituitary.
Additional tests such as free T4, total T4, T3 resin uptake, thyroid
autoantibodies may also be helpful in determining the etiology of
hypothyroidism. More information on specific lab tests can be found under
Diagnostics, Lab Tests (link to the left).
Radioactive iodine uptake (RAIU) and thyroid scanning studies can aid in
assessing the anatomy and function of the thyroid, but are not useful in
assessing hypothyroidism, as they require some level of endogenous function in
the hypofunctioning gland to provide information.
Surgery does not really play a role in primary hypothyroidism. It may
have a role in secondary and tertiary causes, however surgery of this type is
beyond the scope of the general surgeon. Treatment of primary
hypothyroidism typically involves exogenous thyroid hormone replacement, and is
best covered during the medicine rotation.