T3, T4 Synthesis


Lab Tests

Thyroid Scan


Thyroid Cancer



Primary and Secondary Hypothyroidism

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 therapy).

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, and cretinism.

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.

Clinical Presentation


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.