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.

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.

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.

There are several hormones that can be measured in the blood to determine how the thyroid gland is functioning. These include the thyroid hormones triiodothyronine (T3) and its precursor thyroxine (T4), which are produced by the thyroid gland. Thyroid-stimulating hormone (TSH) is another important hormone that is secreted by the anterior pituitary cells in the brain. Its primary function is to increase the production of T3 and T4 by the thyroid gland.

The most useful marker of thyroid gland function is serum thyroid-stimulating hormone (TSH) levels. TSH levels are determined by a classic negative feedback system in which high levels of T3 and T4 suppress the production of TSH, and low levels of T3 and T4 increase the production of TSH. TSH levels are thus often used by doctors as a screening test, where the first approach is to determine whether TSH is elevated, suppressed, or normal.

- Elevated TSH levels can signify inadequate thyroid hormone production (hypothyroidism)

- Suppressed TSH levels can point to excessive thyroid hormone production (hyperthyroidism)

Because a single abnormal TSH level can be misleading, T3 and T4 levels must be measured in the blood to further confirm the diagnosis. When circulating in the body, T3 and T4 are bound to transport proteins. Only a small fraction of the circulating thyroid hormones are unbound or free, and thus biologically active. T3 and T4 levels can thus be measured as free T3 and T4, or total T3 and T4, which takes into consideration the free hormones in addition to the protein-bound hormones. Free T3 and T4 measurements are important because certain drugs and illnesses can affect the concentrations of transport proteins, resulting in differing total and free thyroid hormone levels. There are differing guidelines for T3 and T4 measurements.

- Free T4 levels should be measured in the evaluation of hypothyroidism, and low free T4 establishes the diagnosis. T3 levels are generally not measured in the evaluation of hypothyroidism.

- Free T4 and total T3 can be measured when hyperthyroidism is of high suspicion as it will improve the accuracy of the diagnosis. Free T4, total T3 or both are elevated and serum TSH is below normal in hyperthyroidism. If the hyperthyroidism is mild, only serum T3 may be elevated and serum TSH can be low or may not be detected in the blood.

- Free T4 levels may also be tested in patients who have convincing symptoms of hyper- and hypothyroidism, despite a normal TSH.

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.

Medications to treat hypothyroidism have been found to be safe during pregnancy. Levothyroxine is the treatment of choice for hypothyroidism in pregnancy. Thyroid function should be normalised prior to conception in women with pre-existing thyroid disease. Once pregnancy is confirmed the thyroxine dose should be increased by about 30-50% and subsequent titrations should be guided by thyroid function tests (FT4 and TSH) that should be monitored 4-6 weekly until euthyroidism is achieved. It is recommended that TSH levels are maintained below 2.5 mU/l in the first trimester of pregnancy and below 3 mU/l in later pregnancy. The recommended maintenance dose of thyroxine in pregnancy is about 2.0-2.4 µg/kg daily. Thyroxine requirements may increase in late gestation and return to pre-pregnancy levels in the majority of women on delivery. Pregnant patients with subclinical hypothyroidism (normal FT4 and elevated TSH) should be treated as well, since supplementation with levothyroxine in such cases results in significantly higher delivery rate, with a pooled relative chance of 2.76.

In the developed world, nearly all cases of congenital hypothyroidism are detected by the newborn screening program. These are based on measurement of TSH or thyroxine (T) on the second or third day of life (Heel prick).

If the TSH is high, or the T low, the infant's doctor and parents are called and a referral to a pediatric endocrinologist is recommended to confirm the diagnosis and initiate treatment. Often a technetium (Tc-99m pertechnetate) thyroid scan is performed to detect a structurally abnormal gland. A radioactive iodine (RAIU) exam will help differentiate congenital absence or a defect in organification (a process necessary to make thyroid hormone).

In those without symptoms who are not pregnant there is little evidence for or against screening.

Most children born with congenital hypothyroidism and correctly treated with thyroxine grow and develop normally in all respects. Even most of those with athyreosis and undetectable T levels at birth develop with normal intelligence, although as a population academic performance tends to be below that of siblings and mild learning problems occur in some.

Congenital hypothyroidism is the most common preventable cause of intellectual disability. Few treatments in the practice of medicine provide as large a benefit for as small an effort.

The developmental quotient (DQ, as per Gesell Developmental Schedules) of children with hypothyroidism at age 24 months that have received treatment within the first 3 weeks of birth is summarised below:

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).

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"]."

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.

Hypothyroidism is diagnosed by noting a high TSH associated with a subnormal T4 concentration. Subclinical hypothyroidism (SCH) is present when the TSH is high but the T4 level is in the normal range but usually low normal. SCH is the commonest form of hypothyroidism in pregnancy and is usually due to progressive thyroid destruction due to autoimmune thyroid disease.

Several studies, mostly retrospective, have shown an association between overt hypothyroidism and adverse fetal and obstetric outcomes (e.g. Glinoer 1991). Maternal complications such as miscarriages, anaemia in pregnancy, pre-eclampsia, abruptio placenta and postpartum haemorrhage can occur in pregnant women with overt hypothyroidism. Also, the offspring of these mothers can have complications such as premature birth, low birth weight and increased neonatal respiratory distress. Similar complications have been reported in mothers with subclinical hypothyroidism. A three-fold risk of placental abruption and a two-fold risk of pre-term delivery were reported in mothers with subclinical hypothyroidism. Another study showed a higher prevalence of subclinical hypothyroidism in women with pre-term delivery (before 32 weeks) compared to matched controls delivering at term. An association with adverse obstetrics outcome has also been demonstrated in pregnant women with thyroid autoimmunity independent of thyroid function. Treatment of hypothyroidism reduces the risks of these adverse obstetric and fetal outcomes; a retrospective study of 150 pregnancies showed that treatment of hypothyroidism led to reduced rates of abortion and premature delivery. Also, a prospective intervention trial study showed that treatment of euthyroid antibody positive pregnant women led to fewer rates of miscarriage than non treated controls.

It has long been known that cretinism (i.e. gross reduction in IQ) occurs in areas of severe iodine deficiency due to the fact that the mother is unable to make T4 for transport to the fetus particularly in the first trimester. This neurointellectual impairment (on a more modest scale) has now been shown in an iodine sufficient area (USA) where a study showed that the IQ scores of 7-9 year old children, born to mothers with undiagnosed and untreated hypothyroidism in pregnancy, were seven points lower than those of children of matched control women with normal thyroid function in pregnancy. Another study showed that persistent hypothyroxinaemia at 12 weeks gestation was associated with an 8-10 point deficit in mental and motor function scores in infant offspring compared to children of mothers with normal thyroid function. Even maternal thyroid peroxidase antibodies were shown to be associated with impaired intellectual development in the offspring of mothers with normal thyroid function. Interestingly, it has been shown that it is only the maternal FT4 levels that are associated with child IQ and brain morphological outcomes, as opposed to maternal TSH levels.

Although GH can be readily measured in a blood sample, testing for GH deficiency is constrained by the fact that levels are nearly undetectable for most of the day. This makes simple measurement of GH in a single blood sample useless for detecting deficiency. Physicians therefore use a combination of indirect and direct criteria in assessing GHD, including:

- Auxologic criteria (defined by body measurements)

- Indirect hormonal criteria (IGF levels from a single blood sample)

- Direct hormonal criteria (measurement of GH in multiple blood samples to determine secretory patterns or responses to provocative testing), in particular:

- Subnormal frequency and amplitude of GH secretory peaks when sampled over several hours

- Subnormal GH secretion in response to at least two provocative stimuli

- Increased IGF1 levels after a few days of GH treatment

- Response to GH treatment

- Corroborative evidence of pituitary dysfunction

"Provocative tests" involve giving a dose of an agent that will normally provoke a pituitary to release a burst of growth hormone. An intravenous line is established, the agent is given, and small amounts of blood are drawn at 15 minute intervals over the next hour to determine if a rise of GH was provoked. Agents which have been used clinically to stimulate and assess GH secretion are arginine, levodopa, clonidine, epinephrine and propranolol, glucagon and insulin. An insulin tolerance test has been shown to be reproducible, age-independent, and able to distinguish between GHD and normal adults, and so is the test of choice.

Severe GH deficiency in childhood additionally has the following measurable characteristics:

- Proportional stature well below that expected for family heights, although this characteristic may not be present in the case of familial-linked GH deficiency

- Below-normal velocity of growth

- Delayed physical maturation

- Delayed bone age

- Low levels of IGF1, IGF2, IGF binding protein 3

- Increased growth velocity after a few months of GH treatment

In childhood and adulthood, the diagnosing doctor will look for these features accompanied by corroboratory evidence of hypopituitarism such as deficiency of other pituitary hormones, a structurally abnormal pituitary, or a history of damage to the pituitary. This would confirm the diagnosis; in the absence of pituitary pathology, further testing would be required.

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.

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.

It is often possible to diagnose myxedema on clinical grounds alone. Characteristic symptoms are weakness, cold intolerance, mental and physical slowness, dry skin, typical facies, and hoarse voice. Results of the total serum thyroxine and free thyroxine index tests usually will confirm the diagnosis.

Hormonal assay : there may be low level of T4, TSH, Estrogen, Gonadotropin, Cortisol and ACTH depending on the extent of necrosis

MRI of the pituitary and hypothalamus: this helps to exclude tumor or other pathologies.

This condition is commonly undiagnosed by physicians due to either unfamiliarity with the disease, the subtlety of symptoms, or the attribution of the symptoms to the stresses of having a newborn. Usual screening begins with assessing the thyroid stimulating hormone (TSH) level. A suppressed TSH could represent the hyperthyroid phase, but warrants further testing to investigate for possible Graves' disease. A normal TSH with persistent symptoms could represent the shift between phases and requires repeat testing 4–6 weeks later; an elevated TSH at this time could indicate the hypothyroid phase.

GH deficiency is treated by replacing GH with daily injections under the skin or into muscle. Until 1985, growth hormone for treatment was obtained by extraction from human pituitary glands collected at autopsy. Since 1985, recombinant human growth hormone (rHGH) is a recombinant form of human GH produced by genetically engineered bacteria, manufactured by recombinant DNA technology. In both children and adults, costs of treatment in terms of money, effort, and the impact on day-to-day life, are substantial.

In a study of 1,034 symptomatic adults, Sheehan syndrome was found to be the sixth most frequent etiology of growth hormone deficiency, being responsible for 3.1% of cases (versus 53.9% due to a pituitary tumor).

Primary treatment is prompted by the administration of adequate doses of either the thyroid hormone l-throxine given intravenously or by giving L-triiodothyronine via a nasogastric tube. It is essential to identify and treat the condition precipitating the coma.

Myxedema coma is rare but often fatal. It occurs most often in elderly women and may be mistaken for one of the chronic debilitating diseases common to this age group.

Though the exact cause of myxedema is still unclear, a wealth of skillful research has demonstrated the importance of iodine. In an important study the researchers showed that in the myxedematous type of cretinism treatment with iodine normalizes thyroid function provided that the treatment is begun early in the postnatal period. If not, the prognosis remains dismal.

Thyroid autoantibodies appear mostly with the presence of lymphocytes in the targeted organ. Lymphocytes produce antibodies targeting three different thyroid proteins: Thyroid peroxidase Antibodies (TPOAb), Thyroglobulin Antibodies (TgAb), and Thyroid stimulating hormone receptor Antibodies (TRAb). Some patients who are healthy may be positive for more than one of these antibodies. Doctors who attend to such patients will most likely do routine follow-ups on the patient’s health since, even though it is highly unlikely that they will present any thyroid problems, there is still a chance that they will develop some type of dysfunction with time.

Thyrotoxic myopathy is usually diagnosed by a neurologist who has extensive experience diagnosing neuromuscular disorders. There are many types of neuromuscular disorders that present similar physical symptoms. Extensive clinical tests are performed first to determine if there is a neuromuscular disorder and then to determine which disorder it is. Electromyography is used to diagnose myopathies by comparing muscle contraction responses to electrical stimulus. For TM results may indicate normal responses or myopathic responses depending on how the disorder has progressed. Early detection may indicate normal contractual responses while highly progressed TM may show a significant decrease in contraction response.

Blood tests are then conducted to determine the specific myopathy. For TM, blood tests reveal increased thyroxine levels. Increased thyroxine levels accompanied with decreased neuromuscular responses together provide best evidence for TM diagnosis. Creatine phosphokinase levels are also examined during the blood tests. Normal or increased levels may be observed with TM depending on the severity of TM's progression. Normal levels indicate possible early stages of progression while increased levels may indicate later stages of thyrotoxic myopathy. Muscle biopsies may also be taken and examined to determine TM's progression with respect to physical degradation. Like measured creatine phosphokinase levels results from the muscle biopsy characteristic of TM typically show normal to severe fiber degradation with respective indications to the severity of progression.

Diagnosis is made by imaging/sonography and thyroid hormone tests.