Screening for hypothyroidism is performed in the newborn period in many countries, generally using TSH. This has led to the early identification of many cases and thus the prevention of developmental delay. It is the most widely used newborn screening test worldwide. While TSH-based screening will identify the most common causes, the addition of T testing is required to pick up the rarer central causes of neonatal hypothyroidism. If T determination is included in the screening done at birth, this will identify cases of congenital hypothyroidism of central origin in 1:16,000 to 1:160,000 children. Considering that these children usually have other pituitary hormone deficiencies, early identification of these cases may prevent complications.

In adults, widespread screening of the general population is a matter of debate. Some organizations (such as the United States Preventive Services Task Force) state that evidence is insufficient to support routine screening, while others (such as the American Thyroid Association) recommend either intermittent testing above a certain age in both sexes or only in women. Targeted screening may be appropriate in a number of situations where hypothyroidism is common: other autoimmune diseases, a strong family history of thyroid disease, those who have received radioiodine or other radiation therapy to the neck, those who have previously undergone thyroid surgery, those with an abnormal thyroid examination, those with psychiatric disorders, people taking amiodarone or lithium, and those with a number of health conditions (such as certain heart and skin conditions). Yearly thyroid function tests are recommended in people with Down syndrome, as they are at higher risk of thyroid disease.

Autoantibodies to the thyroid gland may be detected in various disease states. There are several anti-thyroid antibodies, including anti-thyroglobulin antibodies (TgAb), anti-microsomal/anti-thyroid peroxidase antibodies (TPOAb), and TSH receptor antibodies (TSHRAb).

- Elevated anti-thryoglobulin (TgAb) and anti-thyroid peroxidase antibodies (TPOAb) can be found in patients with Hashimoto's thyroiditis, the most common autoimmune type of hypothyroidism. TPOAb levels have also been found to be elevated in patients who present with subclinical hypothyroidism (where TSH is elevated, but free T4 is normal), and can help predict progression to overt hypothyroidism. The American Association Thyroid Association thus recommends measuring TPOAb levels when evaluating subclinical hypothyroidism or when trying to identify whether nodular thyroid disease is due to autoimmune thyroid disease.

- When the etiology of hyperthyroidism is not clear after initial clinical and biochemical evaluation, measurement of TSH receptor antibodies (TSHRAb) can help make the diagnosis. In Grave's disease, TSHRAb levels are elevated as they are responsible for activating the TSH receptor and causing increased thyroid hormone production.

A medical biopsy refers to the obtaining of a tissue sample for examination under the microscope or other testing, usually to distinguish cancer from noncancerous conditions. Thyroid tissue may be obtained for biopsy by fine needle aspiration (FNA) or by surgery.

Fine needle aspiration has the advantage of being a brief, safe, outpatient procedure that is safer and less expensive than surgery and does not leave a visible scar. Needle biopsies became widely used in the 1980s, but it was recognized that the accuracy of identification of cancer was good, but not perfect. The accuracy of the diagnosis depends on obtaining tissue from all of the suspicious areas of an abnormal thyroid gland. The reliability of fine needle aspiration is increased when sampling can be guided by ultrasound, and over the last 15 years, this has become the preferred method for thyroid biopsy in North America.

During pregnancy, the thyroid gland must produce 50% more thyroid hormone to provide enough thyroid hormone for the developing fetus and the expectant mother. In pregnancy, free thyroxine levels may be lower than anticipated due to increased binding to thyroid binding globulin and decreased binding to albumin. They should either be corrected for the stage of pregnancy, or total thyroxine levels should be used instead for diagnosis. TSH values may also be lower than normal (particularly in the first trimester) and the normal range should be adjusted for the stage of pregnancy.

In pregnancy, subclinical hypothyroidism is defined as a TSH between 2.5 and 10 mIU/l with a normal thyroxine level, while those with TSH above 10 mIU/l are considered to be overtly hypothyroid even if the thyroxine level is normal. Antibodies against TPO may be important in making decisions about treatment, and should, therefore, be determined in women with abnormal thyroid function tests.

Determination of TPO antibodies may be considered as part of the assessment of recurrent miscarriage, as subtle thyroid dysfunction can be associated with pregnancy loss, but this recommendation is not universal, and presence of thyroid antibodies may not predict future outcome.

Blood tests may be done prior to or in lieu of a biopsy. The possibility of a nodule which secretes thyroid hormone (which is less likely to be cancer) or hypothyroidism is investigated by measuring thyroid stimulating hormone (TSH), and the thyroid hormones thyroxine (T4) and triiodothyronine (T3).

Tests for serum thyroid autoantibodies are sometimes done as these may indicate autoimmune thyroid disease (which can mimic nodular disease).

Fine Needle Aspiration Cytology (FNAC) is a cheap, simple, and safe method in obtaining cytological specimens for diagnosis by using a needle and a syringe. The "Bethesda System for Reporting Thyroid Cytopathology" is the system used to report whether the thyroid cytological specimen is benign or malignant. It can be divided into six categories:

Repeated FNAC is recommended for Category I, followed by clinical follow-up in Category II, repeat FNAC for Category III, and lobectomy for Category IV, near total-thyroidectomy/lobectomy for Category V, and near total thyroidectomy for Category VI. The risk of malignancy in a malignant FNAC report is 93.7% while for suspicious FNAC report, it is 18.9%.

In overt primary hyperthyroidism, TSH levels are low and T and T levels are high. Subclinical hyperthyroidism is a milder form of hyperthyroidism characterized by low or undetectable serum TSH level, but with a normal serum free thyroxine level. Although the evidence for doing so is not definitive, treatment of elderly persons having subclinical hyperthyroidism could reduce the incidence of atrial fibrillation. There is also an increased risk of bone fractures (by 42%) in people with subclinical hyperthyroidism; there is insufficient evidence to say whether treatment with antithyroid medications would reduce that risk.

The characteristic blood test results for this disorder can also be found in other disorders (for example TSH-oma (pituitary adenoma), or other pituitary disorders). The diagnosis may involve identifying a mutation of the thyroid receptor, which is present in approximately 85% of cases.

Yet, since discovery of resistance to thyroid hormones in the absence of thyroid hormone receptor beta mutations, lack of a mutation in a patient does not rule out resistance.

Measuring the level of thyroid-stimulating hormone (TSH), produced by the pituitary gland (which in turn is also regulated by the hypothalamus's TSH Releasing Hormone) in the blood is typically the initial test for suspected hyperthyroidism. A low TSH level typically indicates that the pituitary gland is being inhibited or "instructed" by the brain to cut back on stimulating the thyroid gland, having sensed increased levels of T and/or T in the blood. In rare circumstances, a low TSH indicates primary failure of the pituitary, or temporary inhibition of the pituitary due to another illness (euthyroid sick syndrome) and so checking the T and T is still clinically useful.

Measuring specific antibodies, such as anti-TSH-receptor antibodies in Graves' disease, or anti-thyroid peroxidase in Hashimoto's thyroiditis — a common cause of hypothyroidism — may also contribute to the diagnosis.

The diagnosis of hyperthyroidism is confirmed by blood tests that show a decreased thyroid-stimulating hormone (TSH) level and elevated T and T levels. TSH is a hormone made by the pituitary gland in the brain that tells the thyroid gland how much hormone to make. When there is too much thyroid hormone, the TSH will be low. A radioactive iodine uptake test and thyroid scan together characterizes or enables radiologists and doctors to determine the cause of hyperthyroidism. The uptake test uses radioactive iodine injected or taken orally on an empty stomach to measure the amount of iodine absorbed by the thyroid gland. Persons with hyperthyroidism absorb much more iodine than healthy persons which includes the radioactive iodine which is easy to measure. A thyroid scan producing images is typically conducted in connection with the uptake test to allow visual examination of the over-functioning gland.

Thyroid scintigraphy is a useful test to characterize (distinguish between causes of) hyperthyroidism, and this entity from thyroiditis. This test procedure typically involves two tests performed in connection with each other: an iodine uptake test and a scan (imaging) with a gamma camera. The uptake test involves administering a dose of radioactive iodine (radioiodine), traditionally iodine-131 (I), and more recently iodine-123 (I). Iodine-123 may be the preferred radionuclide in some clinics due to its more favorable radiation dosimetry (i.e. less radiation dose to the patient per unit administered radioactivity) and a gamma photon energy more amenable to imaging with the gamma camera. For the imaging scan, I-123 is considered an almost ideal isotope of iodine for imaging thyroid tissue and thyroid cancer metastasis.

Typical administration involves a pill or liquid containing sodium iodide (NaI) taken orally, which contains a small amount of iodine-131, amounting to perhaps less than a grain of salt. A 2-hour fast of no food prior to and for 1 hour after ingesting the pill is required. This low dose of radioiodine is typically tolerated by individuals otherwise allergic to iodine (such as those unable to tolerate contrast mediums containing larger doses of iodine such as used in CT scan, intravenous pyelogram (IVP), and similar imaging diagnostic procedures). Excess radioiodine that does not get absorbed into the thyroid gland is eliminated by the body in urine. Some patients may experience a slight allergic reaction to the diagnostic radioiodine and may be given an antihistamine.

The patient returns 24 hours later to have the level of radioiodine "uptake" (absorbed by the thyroid gland) measured by a device with a metal bar placed against the neck, which measures the radioactivity emitting from the thyroid. This test takes about 4 minutes while the uptake % is accumulated (calculated) by the machine software. A scan is also performed, wherein images (typically a center, left and right angle) are taken of the contrasted thyroid gland with a gamma camera; a radiologist will read and prepare a report indicating the uptake % and comments after examining the images. Hyperthyroid patients will typically "take up" higher than normal levels of radioiodine. Normal ranges for RAI uptake are from 10-30%.

In addition to testing the TSH levels, many doctors test for T, Free T, T, and/or Free T for more detailed results. Typical adult limits for these hormones are: TSH (units): 0.45 - 4.50 uIU/mL; T Free/Direct (nanograms): 0.82 - 1.77 ng/dl; and T (nanograms): 71 - 180 ng/dl. Persons with hyperthyroidism can easily exhibit levels many times these upper limits for T and/or T. See a complete table of normal range limits for thyroid function at the thyroid gland article.

In hyperthyroidism CK-MB (Creatine kinase) is usually elevated.

Thyroid hormone resistance syndrome is rare, incidence is variously quoted as 1 in 50,000 or 1 in 40,000 live births. More than 1000 individuals have been identified with thyroid hormone resistance, of which 85% had thyroid hormone beta receptor mutation.

As with hyperthyroidism, TSH is suppressed. Both free and serum (or total) T3 and T4 are elevated. An elevation in thyroid hormone levels is suggestive of thyroid storm when accompanied by signs of severe hyperthyroidism but is not diagnostic as it may also correlate with uncomplicated hyperthyroidism. Moreover, serum T3 may be normal in critically ill patients due to decreased conversion of T4 to T3. Other potential abnormalities include the following:

- Hyperglycemia likely due to catecholamine-mediated effects on insulin release and metabolism as well as increased glycogenolysis, evolving into hypoglycemia when glycogen stores are depleted

- Elevated aspartate aminotransferase (AST), bilirubin and lactate dehydrogenase (LDH)

- Hypercalcemia and elevated alkaline phosphatase due to increased bone resorption

- Elevated white blood cell count

The diagnosis of thyroid storm is based on the presence of symptoms consistent with severe hyperthyroidism, as outlined in the Signs and symptoms section above. Multiple approaches have been proposed to calculate the probability of thyroid storm based on clinical criteria, however, none have been universally adopted by clinicians. For instance, Burch and Wartofsky published the Burch-Wartofsky point scale (BWPS) in 1993, assigning a numerical value based on the presence of specific signs and symptoms organized within the following categories: temperature, cardiovascular dysfunction (including heart rate and presence of atrial fibrillation or congestive heart failure), central nervous system (CNS) dysfunction, gastrointestinal or liver dysfunction and presence of a precipitating event. A Burch-Wartofsky score below 25 is not suggestive of thyroid storm whereas 25 to 45 suggests impending thyroid storm and greater than 45 suggests current thyroid storm. Alternatively, the Japanese Thyroid Association (JTA) criteria, derived from a large cohort of patients with thyroid storm in Japan and published in 2012, provide a qualitative method to determine the probability of thyroid storm. The JTA criteria separate the diagnosis of thyroid storm into definite versus suspected based on the specific combination of signs and symptoms a patient exhibits and require elevated free triiodothyronine (T3) or free thyroxine (T4) for definite thyroid storm.

Detection of any metastases of thyroid cancer can be performed with a full body scintigraphy using iodine-131.

Toxic multinodular goiter can be treated with antithyroid medications such as propylthiouracil or methimazole, radioactive iodine, or with surgery.

Another treatment option is injection of ethanol into the nodules.

Various tests can be chosen depending on the presenting symptoms. Doctors may search for Thyroid peroxidase Antibodies (TPOAb) when a person has symptoms of hypothyroidism, or when a person will be started on a drug therapy associated with risks of developing hypothyroidism, such as lithium or Interferon alfa. This antibody is related to Hashimoto's thyroiditis and Graves' disease. If the person presents symptoms of hyperthyroidism, doctors are more likely to test for Thyroid stimulating hormone receptor Antibodies (TRAb), and monitor the effects of anti-thyroid therapy, also associated with Graves' disease.

Doctors may check Thyroglobulin Antibodies (TgAb) also, whenever a thyroglobulin test is performed to see if the antibody is interfering. TgAb may also be ordered in regular intervals after a person has been diagnosed with thyroid cancer, and just like TPOAb, it can be associated with Hashimoto’s thyroiditis.

Pregnant women who are positive for Hashimoto's thyroiditis may have decreased thyroid function or the gland may fail entirely. If a woman is TPOAb-positive, clinicians can inform her of the risks for themselves and their infants if they go untreated. "Thyroid peroxidase antibodies (TPOAb) are detected in 10% of pregnant women," which presents risks to those pregnancies. Women who have low thyroid function that has not been stabilized are at greater risk of having an infant with: low birth weight, neonatal respiratory distress, hydrocephalus, hypospadias, miscarriage, and preterm delivery. The embryo transplantion rate and successful pregnancy outcomes are improved when Hashimoto's is treated. Recommendations are to only treat pregnant women who are TPOAb-positive throughout the entirety of their pregnancies and to screen all pregnant women for thyroid levels. Close cooperation between the endocrinologist and obstetrician benefits the woman and the infant. The Endocrine Society recommends screening in pregnant women who are considered high-risk for thyroid autoimmune disease.

Thyroid peroxides antibodies testing is recommended for women who have ever been pregnant regardless of pregnancy outcome. "...[P]revious pregnancy plays a major role in development of autoimmune overt hypothyroidism in premenopausal women, and the number of previous pregnancies should be taken into account when evaluating the risk of hypothyroidism in a young women ["sic"]."

Sequence of events:

1. Iodine deficiency leading to decreased T4 production.

2. Induction of thyroid cell hyperplasia due to low levels of T4. This accounts for the multinodular goitre appearance.

3. Increased replication predisposes to a risk of mutation in the TSH receptor.

4. If the mutated TSH receptor is constitutively active, it would then become 'toxic' and produces excess T3/T4 leading to hyperthyroidism.

Cancer staging is the process of determining the extent of the development of a cancer. The TNM staging system is usually used to classify stages of cancers but not of the brain.

Iodine deficiency is treated by ingestion of iodine salts, such as found in food supplements. Mild cases may be treated by using iodized salt in daily food consumption, or drinking more milk, or eating egg yolks, and saltwater fish. For a salt and/or animal product restricted diet, sea vegetables (kelp, hijiki, dulse, nori (found in sushi)) may be incorporated regularly into a diet as a good source of iodine.

The recommended daily intake of iodine for adult women is 150–300 µg for maintenance of normal thyroid function; for men it is somewhat less at 150 µg.

However, too high iodine intake, for example due to overdosage of iodine supplements, can have toxic side effects. It can lead to hyperthyroidism and consequently high blood levels of thyroid hormones (hyperthyroxinemia). In case of extremely high single-dose iodine intake, typically a short-term suppression of thyroid function (Wolff–Chaikoff effect) occurs. Persons with pre-existing thyroid disease, elderly persons, fetuses and neonates, and patients with other risk factors are at a higher risk of experiencing iodine-induced thyroid abnormalities. In particular, in persons with goiter due to iodine deficiency or with altered thyroid function, a form of hyperthyroidism called Jod-Basedow phenomenon can be triggered even at small or single iodine dosages, for example as a side effect of administration of iodine-containing contrast agents. In some cases, excessive iodine contributes to a risk of autoimmune thyroid diseases (Hashimoto's thyroiditis and Graves' disease).

One particular familial form is associated with sensorineural deafness (Pendred's syndrome).

OMIM includes the following:

Graves' disease may present clinically with one of these characteristic signs:

- Rapid heart beat (80%)

- Diffuse palpable goiter with audible bruit (70%)

- Tremor (40%)

- Exophthalmos (protuberance of one or both eyes), periorbital edema (25%)

- Fatigue (70%), weight loss (60%) with increased appetite in young people and poor appetite in the elderly, and other symptoms of hyperthyroidism/thyrotoxicosis

- Heat intolerance (55%)

- Tremulousness (55%)

- Palpitations (50%)

Two signs are truly 'diagnostic' of Graves' disease ("i.e.," not seen in other hyperthyroid conditions): exophthalmos and nonpitting edema (pretibial myxedema). Goiter is an enlarged thyroid gland and is of the diffuse type ("i.e.," spread throughout the gland). Diffuse goiter may be seen with other causes of hyperthyroidism, although Graves' disease is the most common cause of diffuse goiter. A large goiter will be visible to the naked eye, but a small one (mild enlargement of the gland) may be detectable only by physical examination. Occasionally, goiter is not clinically detectable, but may be seen only with computed tomography or ultrasound examination of the thyroid.

Another sign of Graves' disease is hyperthyroidism, "i.e.", overproduction of the thyroid hormones T3 and T4. Normal thyroid levels are also seen, and occasionally also hypothyroidism, which may assist in causing goiter (though it is not the cause of the Graves' disease). Hyperthyroidism in Graves' disease is confirmed, as with any other cause of hyperthyroidism, by measuring elevated blood levels of free (unbound) T3 and T4.

Other useful laboratory measurements in Graves' disease include thyroid-stimulating hormone (TSH, usually undetectable in Graves' disease due to negative feedback from the elevated T3 and T4), and protein-bound iodine (elevated). Serologically detected thyroid-stimulating antibodies, radioactive iodine (RAI) uptake, or thyroid ultrasound with Doppler all can independently confirm a diagnosis of Grave's disease.

Biopsy to obtain histiological testing is not normally required, but may be obtained if thyroidectomy is performed.

The goiter in Graves' disease is often not nodular, but thyroid nodules are also common. Differentiating common forms of hyperthyroidism such as Graves' disease, single thyroid adenoma, and toxic multinodular goiter is important to determine proper treatment. The differentiation among these entities has advanced, as imaging and biochemical tests have improved. Measuring TSH-receptor antibodies with the h-TBII assay has been proven efficient and was the most practical approach found in one study.

The diagnostic workup of a suspected iodine deficiency includes signs and symptoms as well as possible risk factors mentioned above. A 24-hour urine iodine collection is a useful medical test, as approximately 90% of ingested iodine is excreted in the urine. For the standardized 24-hour test, a 50 mg iodine load is given first, and 90% of this load is expected to be recovered in the urine of the following 24 hours. Recovery of less than 90% is taken to mean high retention, that is, iodine deficiency. The recovery may, however, be well less than 90% during pregnancy, and an intake of goitrogens can alter the test results.

If a 24-hour urine collection is not practical, a random urine iodine-to-creatinine ratio can alternatively be used. However, the 24-hour test is found to be more reliable.

A general idea of whether a deficiency exists can be determined through a functional iodine test in the form of an iodine skin test. In this test, the skin is painted with an iodine solution: if the iodine patch disappears quickly, this is taken as a sign of iodine deficiency. However, no accepted norms exist on the expected time interval for the patch to disappear, and in persons with dark skin color the disappeance of the patch may be difficult to assess. If a urine test is taken shortly after, the results may be altered due to the iodine absorbed previously in a skin test.

Diagnosis is usually made by detecting elevated levels of anti-thyroid peroxidase antibodies (TPOAb) in the serum, but seronegative (without circulating autoantibodies) thyroiditis is also possible.

Given the relatively non-specific symptoms of initial hypothyroidism, Hashimoto's thyroiditis is often misdiagnosed as depression, cyclothymia, PMS, chronic fatigue syndrome, fibromyalgia and, less frequently, as erectile dysfunction or an anxiety disorder. On gross examination, there is often presentation of a hard goiter that is not painful to the touch; other symptoms seen with hypothyroidism, such as periorbital myxedema, depend on the current state of progression of the response, especially given the usually gradual development of clinically relevant hypothyroidism. Testing for thyroid-stimulating hormone (TSH), free T3, free T4, and the anti-thyroglobulin antibodies (anti-Tg), anti-thyroid peroxidase antibodies (anti-TPO, or TPOAb) and anti-microsomal antibodies can help obtain an accurate diagnosis. Earlier assessment of the person may present with elevated levels of thyroglobulin owing to transient thyrotoxicosis, as inflammation within the thyroid causes damage to the integrity of thyroid follicle storage of thyroglobulin; TSH secretion from the anterior pituitary increases in response to a decrease in negative feedback inhibition secondary to decreased serum thyroid hormones. Typically T4 is the preferred thyroid hormone test for hypothyroidism. This exposure of the body to substantial amounts of previously isolated thyroid enzymes is thought to contribute to the exacerbation of tolerance breakdown, giving rise to the more pronounced symptoms seen later in the disease. Lymphocytic infiltration of the thyrocyte-associated tissues often leads to the histologically significant finding of germinal center development within the thyroid gland.

Hashimoto's when presenting as mania is known as Prasad's syndrome after Ashok Prasad, the psychiatrist who first described it.

The most common and helpful way to diagnose thyroiditis is first for a physician to palpate the thyroid gland during a physical examination. Laboratory tests allow doctors to evaluate the patient for elevated erythrocyte sedimentation rates, elevated thyroglobulin levels, and depressed radioactive iodine uptake (Mather, 2007). Blood tests also help to determine the kind of thyroiditis and to see how much thyroid stimulating hormone the pituitary gland is producing and what antibodies are present in the body. In some cases a biopsy may be needed to find out what is attacking the thyroid.

Most patients with thyroid adenoma can be managed by watchful waiting (without surgical excision) with regular monitoring. However, some patients still choose surgery after being fully informed of the risks. Regular monitoring mainly consists of watching for changes in nodule size and symptoms, and repeat ultrasonography or needle aspiration biopsy if the nodule grows.