Additional Laboratory Tests for TSH Evaluation
When TSH is abnormal, always measure free T4 to distinguish between subclinical and overt thyroid dysfunction, and consider anti-TPO antibodies if autoimmune thyroiditis is suspected. 1, 2
Essential Follow-Up Tests Based on TSH Results
For Elevated TSH (>4.5 mIU/L)
Free T4 (fT4) is mandatory to differentiate subclinical hypothyroidism (normal fT4) from overt hypothyroidism (low fT4), as this distinction fundamentally changes management 2, 3.
Anti-thyroid peroxidase (anti-TPO) antibodies should be measured to confirm autoimmune etiology, which predicts 4.3% annual progression to overt hypothyroidism versus 2.6% in antibody-negative patients 2.
Lipid panel is reasonable, as subclinical hypothyroidism may affect cholesterol levels and treatment with levothyroxine may lower LDL cholesterol 2.
Repeat TSH in 3-6 weeks before initiating treatment, since 30-60% of elevated TSH values normalize spontaneously 2, 4.
For Suppressed TSH (<0.1 mIU/L)
Free T4 and free T3 (fT3) are both required to determine if the patient has overt hyperthyroidism (elevated thyroid hormones) or subclinical hyperthyroidism (normal thyroid hormones) 1, 5, 3.
In patients on levothyroxine, these tests distinguish between appropriate TSH suppression for thyroid cancer versus iatrogenic hyperthyroidism requiring dose reduction 2.
Free T3 is particularly important when hyperthyroidism is suspected but fT4 is normal, as T3 toxicosis presents with isolated fT3 elevation 3.
For Patients on Levothyroxine Therapy
Both TSH and free T4 should be measured together during dose titration every 6-8 weeks, as fT4 helps interpret ongoing abnormal TSH levels since TSH may take longer to normalize 2, 6.
Once stable, monitor TSH and fT4 every 6-12 months or when symptoms change 2.
Free T3 measurement is generally not necessary for routine monitoring of levothyroxine therapy, as T4 monotherapy adequately maintains thyroid status in most patients 5, 6.
Special Clinical Scenarios Requiring Additional Testing
Suspected Central Hypothyroidism
Free T4 must be measured alongside TSH in patients with pituitary disease, as TSH may be inappropriately normal or only mildly elevated despite low fT4 1, 2.
Check early morning ACTH and cortisol before starting thyroid hormone replacement, as initiating levothyroxine before corticosteroids can precipitate adrenal crisis in patients with concurrent adrenal insufficiency 1, 2.
Consider other pituitary hormones (LH, FSH, IGF-1) if hypophysitis is suspected, particularly in patients on immune checkpoint inhibitors 1.
Patients on Immune Checkpoint Inhibitors
TSH should be checked before each cycle for the first 3 months, then every second cycle thereafter, with fT4 added if TSH is abnormal 1, 2.
Glucose and HbA1c should be monitored, as new-onset diabetes can occur with immunotherapy 1.
If central hypothyroidism is suspected, measure early morning ACTH and cortisol to exclude hypophysitis 1.
Pregnancy or Planning Pregnancy
TSH and free T4 should both be measured, as inadequate treatment during pregnancy increases risk of preeclampsia, low birth weight, and neurodevelopmental effects 2.
Target TSH should be maintained in the low-normal range (0.5-2.5 mIU/L) during pregnancy 2.
Levothyroxine requirements typically increase 25-50% during early pregnancy, necessitating more frequent monitoring 2.
Thyroid Cancer Patients
TSH, free T4, and thyroglobulin (Tg) should be measured at follow-up visits 1.
Thyroglobulin antibodies must be checked simultaneously with Tg, as their presence interferes with Tg measurement 1.
Target TSH varies by risk stratification: 0.5-2 mIU/L for low-risk patients, 0.1-0.5 mIU/L for intermediate-risk, and <0.1 mIU/L for high-risk or persistent disease 1, 2.
Tests That Are NOT Routinely Necessary
Total T4 and total T3 measurements have been superseded by free hormone measurements and are only useful in research settings or severe hyperthyroidism 5, 3.
Reverse T3 (rT3) is not recommended for routine clinical use, as it does not change management in most situations 4.
TRH stimulation testing has been replaced by sensitive TSH assays and is no longer necessary for routine thyroid assessment 6.
Critical Pitfalls to Avoid
Never treat based on a single abnormal TSH value without confirmation testing, as transient elevations from acute illness, medications, or recovery from thyroiditis are common 2, 4.
Do not check thyroid function during acute illness unless thyroid storm or myxedema coma is suspected, as non-thyroidal illness causes artifactual abnormalities in 16-25% of hospitalized patients 4.
Always measure free T4 when TSH is abnormal rather than relying on calculated indices (free T4 index), as direct free hormone measurements have superior diagnostic performance 5, 3.
In patients with suspected central hypothyroidism, never start levothyroxine before ruling out adrenal insufficiency, as this can precipitate life-threatening adrenal crisis 1, 2.