What are the initial labs for thyroid function tests and tuberculosis diagnosis?

Initial Laboratory Testing for TLS (Thyroid and Tuberculosis)

Thyroid Function Testing

For initial thyroid evaluation, order TSH as the first-line test, followed by free T4 if TSH is abnormal. 1, 2

• TSH (thyroid-stimulating hormone) is the preferred initial screening test for suspected primary hypothyroidism 2
• If TSH is elevated or suppressed, obtain free T4 (thyroxine) to narrow the diagnosis 1, 2
• The combination of TSH and free T4 allows calculation of the free thyroxine index (FTI), which approximates free T4 in serum 1
• Obtain T3 (triiodothyronine) level only if TSH is undetectable and free T4 is normal, suggesting hyperthyroidism 2

Important Caveat for Central Hypothyroidism

• If you suspect pituitary or hypothalamic dysfunction (central hypothyroidism), TSH cannot be used for monitoring—instead use free T4 and T3 concentrations directly 2
• Central hypothyroidism presents with low TSH and low free T4, unlike primary hypothyroidism where TSH is elevated 3

Tuberculosis Diagnostic Testing

For TB diagnosis, obtain three sputum specimens for AFB smear microscopy, culture, and molecular testing (GeneXpert MTB/RIF), with the molecular test performed on at least the first specimen. 4, 5

Bacteriological Confirmation (Preferred)

• Collect three sputum specimens for comprehensive testing—the first specimen has 53.8% sensitivity, increasing to approximately 70% with three specimens 5
• AFB (acid-fast bacilli) smear microscopy should be performed on all specimens, preferably using fluorescence microscopy on concentrated specimens 5
• GeneXpert MTB/RIF (molecular testing) should be performed on at least the first diagnostic specimen, with results available within 48 hours 4, 5
• Mycobacterial culture remains the gold standard, with liquid cultures having 88-90% sensitivity compared to 76% for solid cultures 5
• Drug susceptibility testing for isoniazid, rifampin, and ethambutol should be performed on all positive initial cultures 4

IGRA Testing for TB Infection

• IGRA (interferon-gamma release assay) tests such as QuantiFERON or T-SPOT are preferred over tuberculin skin testing for detecting TB infection 4
• IGRAs do not distinguish latent TB infection from active TB disease—they only indicate infection with Mycobacterium tuberculosis 4
• A negative IGRA does not exclude TB in high-risk patients or those with clinical suspicion of active disease 4

Additional TB Workup

• Chest CT scan is recommended when TB is suspected, as it reveals abnormalities not visible on plain radiography 4, 6
• For patients unable to produce sputum, consider sputum induction with hypertonic saline or bronchoscopy under appropriate infection control measures 4
• In children, obtain gastric aspirates as alternative specimens 5

Baseline Testing Before TB Treatment

Prior to initiating TB therapy, obtain baseline liver function tests (AST, ALT, bilirubin, alkaline phosphatase), serum creatinine, platelet count, and visual acuity testing if ethambutol will be used. 4

• Hepatic function tests (AST, ALT, bilirubin, alkaline phosphatase) are essential baseline measurements 4
• Serum creatinine and platelet count should be obtained before treatment 4
• Visual acuity and red-green color discrimination testing are required when ethambutol is planned 4
• HIV testing with counseling is recommended for all TB patients, with CD4 count if HIV-positive 4
• Consider hepatitis B and C serologic testing for patients with risk factors (injection drug use, foreign birth in Asia/Africa, HIV infection) 4

Monitoring During TB Treatment

• Monthly sputum specimens for microscopy and culture until two consecutive specimens are culture-negative 4
• Monthly clinical evaluations to assess adherence and identify adverse effects 4
• For children on second-line TB drugs, monitor TSH and T4 at baseline, 3 months, then every 3 months if on ethionamide, prothionamide, or PAS 4
• Renal function (creatinine and potassium) should be monitored monthly while on injectable agents 4

Special Consideration: Thyroid Tuberculosis

• When thyroid tuberculosis is suspected (rare presentation with thyroid nodule or goiter), fine-needle aspiration cytology (FNAC) with culture is the diagnostic method of choice 7, 8
• Monitor thyroid function in confirmed thyroid TB, as it can cause transient hyperthyroidism followed by hypothyroidism requiring replacement therapy 7