Serum thyroid-stimulating hormone (TSH) is the most useful thyroid function screening for primary disorders of the thyroid (i.e., primary hyper- and hypothyroidism).

TSH is of no value in the diagnosis and management of secondary (hypothalamic-pituitary) hypothyroidism; free thyroxine index (FTI) or free T₄ (FT4) are more useful tests.

Primary disorders of the thyroid gland are far more common than are secondary.

The majority of thyroid hormone (both T₃ and T₄) is protein bound. Factors that affect protein levels will affect total hormone levels.

Thyroid function tests are readily interpretable in ambulatory patients, but are often not helpful or may be confusing in the hospitalized sick patient.

Population screening for thyroid dysfunction is not recommended except in the neonate (universal screening) and possibly in pregnancy.

Hypothyroidism is the single most common cause of abnormal thyroid function.

TSH >10 to 20 µU/mL is generally diagnostic of primary hypothyroidism.

Levothyroxine (LT₄) is the treatment of choice for hypothyroidism.

Maintaining the same brand-name levothyroxine preparation is recommended by specialty groups when treating hypothyroidism since significant differences in bioavailability are known to be present in FDA-approved generic interchanges.

In secondary hypothyroidism, it is important to treat adrenal insufficiency, if present, before thyroid replacement.

The clinical diagnosis of hyperthyroidism may prove difficult in the elderly.

If not contraindicated, a biochemical diagnosis of suppressed TSH and elevated circulating T₄ or T₃ requires a radioactive iodine uptake (RAIU) test or thyroid scan to confirm the diagnosis of hyperthyroidism and aid in the treatment plan.

Graves’ disease is the most common cause of non-iatrogenic hyperthyroidism and is associated with many other autoimmune illnesses.

Patients with toxic multinodular goiter typically present at an older age (>6th decade) than those with Graves’ disease.

Jod-Basedow is the only form of thyrotoxicosis in which ongoing overproduction of thyroid hormone by the thyroid gland occurs associated with a low RAIU.

Half of patients with De Quervain’s thyroiditis present with symptoms of hyperthyroidism. Typical findings include an extremely elevated ESR, suppressed TSH, and low RAIU.

Methimazole is the preferred antithyroid drug except during the first trimester of pregnancy when propylthiouracil is recommended.

Agranulocytosis is a rare but potentially fatal side effect of antithyroid medications. All patients taking these medications must be counseled to seek medical attention and obtain a white blood cell count in the face of fevers or sore throat.

¹³¹I therapy can be used to treat hyperthyroid conditions caused by thyroid hormone overproduction, including Graves’ disease, toxic multinodular goiter, and toxic adenoma.

Thyroid nodules are common, and most often benign.

Fine needle aspiration (FNA) or biopsy is the diagnostic test of choice in evaluating a thyroid nodule.

Thyroid scans are generally not indicated for the evaluation of nonfunctional thyroid nodules.

Risk factors for thyroid cancer include male gender, prior history of head/neck radiation, presence of a dominant nodule, and positive family history.

Papillary and follicular variants are the most common causes of thyroid cancer.

Papillary cancer tends to metastasize to lymph nodes, whereas follicular cancer tends to have earlier hematogeneous spread, primarily to lung and bone.

Treatment of thyroid cancer includes surgery, ablative ¹³¹I therapy, and LT₄ suppression of TSH.