Hypothyroidism

Indications for Testing

Laboratory testing for hypothyroidism is appropriate in individuals with:

• Signs or symptoms of hypothyroidism
• A family history of autoimmune thyroiditis
• Thyroid nodules

Laboratory testing may also be used to monitor the progression of disease and to guide treatment. This ARUP Consult topic discusses laboratory testing for hypothyroidism in nonpregnant adults, newborns, and children; for information on testing in pregnancy, see the ARUP Consult Thyroid Disease in Pregnancy topic.

Laboratory Testing in Nonpregnant Adults

Diagnosis

Initial Evaluation

Hypothyroidism is typically marked by elevated TSH concentrations. Following an abnormal TSH test result, a serum free thyroxine (T4) test is recommended.  Total T4 testing is not recommended for the initial evaluation of hypothyroidism.  Subclinical hypothyroidism is marked by an elevated TSH concentration with a normal free T4 concentration, whereas overt hypothyroidism is marked by an elevated TSH concentration with a low free T4 concentration.  Central hypothyroidism (insufficient production of TSH due to a pituitary or hypothalamic disorder) may be marked by low or normal TSH and, usually, low T4 concentrations; therefore, free T4 testing may be more useful than TSH testing.  Triiodothyronine (T3) testing is not recommended for the diagnosis of hypothyroidism,  although it will be low in central hypothyroidism.

For more information on thyroid function tests in hypothyroidism, see the ARUP Consult Initial Evaluation of Thyroid Function topic.

Autoantibody Tests

Autoantibody testing is useful to distinguish autoimmune thyroiditis from other etiologies of thyroid disease. For example, it is recommended that a thyroid peroxidase (TPO) autoantibody (TPOAb) titer be considered in patients with subclinical hypothyroidism based on elevated TSH and normal serum free T4 concentrations to identify autoimmune disease.   TPOAb testing can also be used to confirm the diagnosis of overt hypothyroidism and to predict the risk of progression to overt hypothyroidism in patients with autoimmune and other disorders.  For more information on other thyroid autoantibodies and autoantibody testing in thyroid disease, see the ARUP Consult Autoimmune Thyroiditis topic.

Other Tests and Procedures

A radioactive iodine uptake test and scan can be used to differentiate between etiologies of thyroid disease in nonpregnant adults.  Ultrasound may be a safe and cost-effective alternative to radioactive testing; it is recommended in patients with amiodarone-induced thyrotoxicosis, as well as in patients who are lactating or pregnant (see the ARUP Consult Thyroid Disease in Pregnancy topic).  Otherwise, routine ultrasound is not recommended unless there is a palpable thyroid abnormality. 

Fine needle aspiration (FNA) is recommended for patients with severe thyroid pain and systemic symptoms to investigate the potential etiology of thyroid disease.  FNA is also used to evaluate patients for suspected thyroid cancer (see the ARUP Consult Thyroid Cancer topic).

Treatment Monitoring

Biochemical measurements should be used to guide therapy with levothyroxine, the medicinal form of T4, in patients with hypothyroidism. The serum TSH concentration is the most reliable marker of therapeutic efficacy,  although serum free T4 is sometimes measured.  A TSH value within the appropriate reference interval should be the therapeutic target for levothyroxine treatment.  Tests for total or free T3 concentrations do not reflect intracellular T3 concentrations or clinical response and are not recommended.  Testing is recommended 4-6 weeks after initiation of therapy or after a change in dose.  Once the appropriate dose of levothyroxine has been reached, TSH measurements are recommended after 6 months and then at 12-month intervals, or more frequently, depending on the clinical situation.  Repeat thyroid function testing is recommended for patients who change levothyroxine brands because formulations are not necessarily therapeutically equivalent.   Measurement of serum TSH is also recommended 4-8 weeks after initiating treatment with any drug that affects the bioavailability or function of levothyroxine, and every 4-8 weeks thereafter until a stable euthyroid state is reached. 

Laboratory Testing in Newborns

Screening

Universal neonatal screening for congenital hypothyroidism is performed to prevent neurodevelopmental disability and improve outcomes.     Different countries have different guidelines for neonatal screening. In the U.S., screening strategies include primary TSH testing with backup T4 testing if TSH is elevated, primary T4 testing with backup TSH testing if T4 is low, or combined primary TSH and T4 testing.  A high-quality dried capillary blood spot is the preferred specimen.  Screening is recommended at age 2-4 days.  Earlier testing (in neonates younger than 48 hours of age) may lead to falsely elevated TSH concentrations. 

Diagnosis

Initial Evaluation

If the results of newborn screening tests suggest congenital hypothyroidism, follow-up confirmatory free T4 and TSH testing with a venous blood sample (not a repeat heel prick) is recommended.   A high TSH concentration with a low free T4 concentration is indicative of congenital hypothyroidism.   A low free T4 result, regardless of the TSH concentration, or a persistently very high TSH result, regardless of the T4 concentration, also suggests hypothyroidism and may warrant treatment.  Imaging may be necessary to determine if there is a structural cause of congenital hypothyroidism. 

Serum thyroglobulin (Tg) testing, in combination with a clinical exam, thyroid ultrasound, and scintigraphy, may help to elucidate the etiology of congenital hypothyroidism.  For example, an undetectable serum Tg concentration strongly suggests thyroid dysgenesis as the etiology of hypothyroidism. For more information on Tg in thyroid disease, see the ARUP Consult Autoimmune Thyroiditis topic.

Treatment Monitoring

The biochemical goal of treatment with levothyroxine is to normalize serum T4 and TSH concentrations.  In infants being treated with levothyroxine, follow-up testing should occur 1-2 weeks after the start of treatment, every 2 weeks until the TSH concentration has normalized, and every 1-2 months thereafter until the infant reaches 12 months of age.   TSH and free T4 are the recommended tests.   Samples should be collected at least 4 hours after the last dose is administered.  An elevated TSH concentration with a low T4 concentration on a repeat test 4-6 weeks after the cessation of levothyroxine therapy establishes permanent congenital hypothyroidism. 

Laboratory Testing in Children

Screening

Routine thyroid function tests in children with obesity are not recommended.   Although TSH concentrations may be elevated in children with obesity, these elevations rarely represent true hypothyroidism, and free T4 concentrations are generally normal.   It is recommended that testing be considered only in children with small stature or low height velocity for their stage of puberty.  

Treatment Monitoring

Laboratory and clinical evaluations are recommended every 2-4 months for children 1-3 years of age who are taking levothyroxine for hypothyroidism, and every 3-12 months thereafter until adulthood.  The frequency of evaluation should decrease as the child ages,  although more frequent evaluation is recommended if treatment noncompliance is suspected or if laboratory tests yield abnormal values.  Testing is also recommended 4-6 weeks after any change in levothyroxine formulation or dose.  Measurement of TSH and free T4 is recommended.   Samples should be collected at least 4 hours after the last dose is administered.