Thyroid function tests

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Furqan M M. M.B.B.S[2]

Synonyms and keywords: TFTs

Overview

Thyroid function tests (TFTs) is a collective term for blood tests used to check the function of the thyroid. A TFT panel typically includes thyroid-stimulating hormone (TSH, thyrotropin) and thyroxine (T4), and triiodothyronine (T3) depending on local laboratory policy.TFTs may be requested if a patient is thought to suffer from hyperthyroidism (overactive thyroid) or hypothyroidism (underactive thyroid) or to monitor the effectiveness of either thyroid-suppression or hormone replacement therapy. It is also requested routinely in conditions linked to thyroid disease, such as atrial fibrillation. The normal adult reference range for serum TSH is 0.4-4.5 mIU/L. The adult reference range of FT4 using comparative direct methods is 0.7-1.8 ng/dL (9-23 pmol/L) and that for FT3 is 2.3-5.0 pg/mL (35-77 pmol/L). A high TSH level may indicate hypothyroidism. If the results indicate high TSH levels, a second confirmatory sample including TSH and free T4 levels should be tested prior to starting treatment. TSH should not be measured in isolation in patients with suspected hypothalamic-pituitary disease; the measurement of free T4 is essential. Free T4 testing, combined with TSH testing, is recommended for the routine assessment and diagnosis of thyroid function and following hyperthyroidism treatment and may or may not be used in cases of hypothyroidism.

Thyroid function tests

Thyroid function tests serve as an important diagnostic tool in the diagnosis and follow-up of various hyper and hypothyroid states:

• The normal adult reference range for serum TSH is 0.4-4.5 mIU/L. A high TSH level may indicate hypothyroidism. If the results indicate high TSH levels, a second confirmatory sample including TSH and free T4 levels should be tested prior to starting treatment.
• The adult reference range of FT4 using comparative direct methods is 0.7-1.8 ng/dL (9-23 pmol/L) and that for FT3 is 2.3-5.0 pg/mL (35-77 pmol/L).
• TSH should not be measured in isolation in patients with suspected hypothalamic-pituitary disease; the measurement of free T4 is essential. Free T4 testing, combined with TSH testing, is recommended for the routine assessment and diagnosis of thyroid function and following hyperthyroidism treatment and may or may not be used in cases of hypothyroidism.
• Serum T3 testing has a low sensitivity and specificity for diagnosing hypothyroidism.
• Abnormal levels of total T4 and T3 often result from changes in transport proteins, not thyroid function. Total hormone testing should only be performed when there are discrepancies in the free hormone levels.
• Conditions other than thyroid disease should be considered when TSH levels are abnormal such as physiological changes associated with pregnancy, severe nonthyroidal illness, and medication use (including amiodarone, lithium, dopamine, and glucocorticoids).

Differential Diagnosis

Increased TSH levels

• Hashimoto's thyroiditis
• Riedel's thyroiditis
• De Quervain's thyroiditis

Decreased TSH levels

• Graves disease
• Thyrotoxicosis
  • De Quervain's thyroiditis
  • Hashimoto's thyroiditis
• Amiodarone

Thyroid function tests in hypothyroidism

• Thyroid function tests may differentiate various causes of hypothyroidism:

Disease		History and symptoms		Laboratory findings							Additional findings
		Fever	Pain	TSH	Free T4	T3	T3RU†	Thyroglobin	TRH	TPOAb^
Primary hypothyroidism	Autoimmune (Hashimoto's thyroiditis)	-	-	↑*	↓	Normal/↓	Normal/↓	Normal/↑	Normal	Present (high titer)	May be accompanied by other autoimmune diseases
	Riedel's thyroiditis	-	-	Normal/↑	Normal/↓	Normal/↓	Normal/↓	Normal	Normal	Usually present	Riedel's thyroiditis usually presents with hard and fixed thyroid mass.
	Infectious thyroiditis	+	+	Normal	Normal	Normal	Normal	Normal	Normal	Absent	Infectious thyroiditis associated with neck pain
Transient hypothyroidism	Subacute (de Quervain's) thyroiditis	+/-	+/-	↑/↓	↓/↑	Normal	↓	↑	Normal	Low/absent	May present primarily with hyperthyroidism
	Postpartum thyroiditis	+/-	+/-	↑/↓	↓/↑	Normal	↓	↑	Normal/↑	Present (high titer)	May present primarily with hyperthyroidism
	Silent thyroiditis	-	-	↑/↓	↓/↑	Normal	↓	↑	Normal	Present (high titer)	May present primarily with hyperthyroidism
Others	Drug-induced	-	-	↑/↓	↓/↑	Normal	↓	Normal/↑	Normal	Absent**	History of hyperthyroidism History of trauma History of drug use, surgery, or radiation
	Radiation-induced
	Trauma induced
	Radioiodine induced
	Thyroidectomy
	Subclinical hypothyroidism	-	-	↑	Normal	Normal	Normal	Normal	Normal	Normal/↑	Asymptomatic

(†)T3RU; Triiodothyronine Resin uptake. (^)TPOAb; Thyroid peroxidase antibodies. (*)TSH may be decreased transiently in the thyrotoxicosis. (**)TPOAb may be present in drug-induced hypo/hyperthyroidism such as Interferon-alpha, interleukin-2, and lithium.

Thyroid function tests in hyperthyroidism

• Thyroid function tests may differentiate various causes of hyperthyroidism:

Disease		History and symptoms		Laboratory findings								Additional findings
		Fever	Pain	TSH	Free T4	T3	T3RU†	Thyroglobin	TRH	TSH Receptor Antibody	TPOAb^
Thyroiditis	Hashimoto's thyroiditis (Hashitoxicosis)	-	-	↑*	↓	Normal/↓	Normal/↓	Normal/↑	Normal	Absent	Present (high titer)	May be accompanied by other autoimmune diseases
	Subacute (de Quervain's) thyroiditis	+/-	+/-	↑/↓	↓/↑	Normal	↓	↑	Normal	Absent	Low/absent	May present with hypothyroidism
	Postpartum thyroiditis	+/-	+/-	↑/↓	↓/↑	Normal	↓	↑	Normal/↑	Absent	Present (high titer)	May present with hypothyroidism
	Silent thyroiditis	-	-	↑/↓	↓/↑	Normal	↓	↑	Normal	Absent	Present (high titer)	May present with hypothyroidism
Primary hyperthyroidism	Grave's disease	-	-	↓	↑	Normal/↑	↑	↑	Normal	Present	Absent	Patient may have opthalmopathy and dermopathy
	Toxic thyroid nodule	-	-	↓	↑	Normal/↑	↑(hot nodule)	Normal/↑	Normal	Absent	Absent	-
Secondary hyperthyroidism	Pituitary adenoma	-	-	↑	↑	Normal/↑	↑	Normal/↑	Normal	Absent	Absent	Inappropriately normal or increased TSH
Tertiary hyperthyroidism	Tertiary hyperthyroidism	-	-	↑	↑	↑	↑	Normal/↑	↑	Absent	Absent	Inappropriately normal or increased TSH
Drug induced	Amiodarone type 1	-	-	↓	↑	Normal/↑	↓	Normal/↑	Normal	Absent	Absent	High urinary iodine
	Amiodarone type 2	-	-	↓	↑	Normal/↑	Absent/↓	Normal/↑	Normal	Absent	Absent	High urinary iodine
Others	Factitious thyrotoxicosis	-	-	↓	↑	Normal/↑	↓	↓	Normal	Absent	Absent	Decreased thyroglobulin
	Trophoblastic disease	-	-	↓	↑	Normal/↑	↑	-	Normal	Absent	Absent	-
	Struma ovarii	-	-	↓	↑	Normal/↑	↓	-	Normal	Absent	Absent	-

(†)T3RU; Triiodothyronine Resin uptake. (^)TPOAb; Thyroid peroxidase antibodies.

References

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