Laboratory Testing for Hypothyroidism

Measure serum TSH as the initial screening test, followed by free T4 if TSH is abnormal, to diagnose hypothyroidism. 1, 2, 3, 4

Primary Diagnostic Tests

TSH is the single most sensitive and specific test for detecting primary hypothyroidism, with sensitivity above 98% and specificity greater than 92% 1, 2, 4
An elevated TSH level almost always signals primary hypothyroidism and should prompt further evaluation 4
Normal reference range is typically 0.4-4.5 mIU/L, though this varies by laboratory and shifts upward with age 1, 2

Measure free T4 when TSH is abnormal to distinguish between subclinical hypothyroidism (normal free T4) and overt hypothyroidism (low free T4) 1, 2, 3, 4
This combination differentiates the severity of thyroid dysfunction and guides treatment decisions 1, 4
In overt primary hypothyroidism, TSH is elevated and free T4 is below the reference range 5, 4
In subclinical hypothyroidism, TSH is elevated but free T4 remains within normal limits 6, 4

Obtain T3 level only if TSH is undetectable and free T4 is normal, which may indicate T3 toxicosis 3
T3 measurement is not routinely needed for diagnosing hypothyroidism 3

Repeat TSH and free T4 after 3-6 weeks to confirm abnormal results before initiating treatment, as 30-60% of elevated TSH values normalize spontaneously 1, 2
TSH levels can vary up to 50% day-to-day due to pulsatile secretion and physiological factors 1, 2
Never base treatment decisions on a single abnormal TSH value 1

Measure anti-thyroid peroxidase (anti-TPO) antibodies and anti-thyroglobulin antibodies to confirm autoimmune thyroiditis (Hashimoto's disease), the most common cause of primary hypothyroidism 2, 6
Positive anti-TPO antibodies predict higher risk of progression to overt hypothyroidism (4.3% per year vs 2.6% in antibody-negative patients) 1
In children with type 1 diabetes, measure these antibodies shortly after diabetes diagnosis due to increased autoimmune risk 2

Measure both TSH and free T4 when hypothyroidism is suspected in pregnant women 2
Treatment is critical in pregnancy to prevent adverse outcomes including preeclampsia, low birth weight, and neurodevelopmental effects in offspring 1

In suspected central hypothyroidism, TSH cannot be used as the primary screening test because TSH may be low, normal, or even slightly elevated despite low thyroid hormone levels 5, 3
Measure free T4 and T3 concentrations directly, which should be maintained in the upper half of the normal range during treatment 6, 3
Evaluate other pituitary hormones, especially the hypothalamic-pituitary-adrenal axis, as hypocortisolism must be corrected before starting thyroid hormone replacement 6, 5

Acute illness, hospitalization, certain medications (iodine, dopamine, glucocorticoids), adrenal insufficiency, pregnancy, and pituitary disorders can all affect TSH levels 2
Recent iodine exposure (such as from CT contrast) can transiently alter thyroid function tests 1
Recovery phase from thyroiditis may show temporarily elevated TSH 1

Approximately 12% of people over age 80 without thyroid disease have TSH >4.5 mIU/L, indicating that standard reference ranges may not apply to elderly patients 1, 2
TSH levels vary by age, sex, and race/ethnicity 2

Do not rely on a single TSH measurement due to substantial day-to-day variability; always confirm with repeat testing 1, 2
Do not screen asymptomatic individuals without risk factors, as the prevalence of overt hypothyroidism in the general population is low (0.3%) and screening may lead to unnecessary treatment 2, 7
Do not measure T3 routinely in suspected hypothyroidism, as it adds little diagnostic value in primary hypothyroidism 3
In central hypothyroidism, do not use TSH to monitor treatment adequacy; use free T4 and T3 levels instead 6, 3
Always rule out adrenal insufficiency before starting thyroid hormone replacement in central hypothyroidism to prevent adrenal crisis 6, 5