From the Guidelines
The diagnostic workup for hypothyroidism should begin with thyroid function tests, specifically thyroid-stimulating hormone (TSH) and free thyroxine (T4) levels, as recommended by the most recent guidelines 1. The initial evaluation should include TSH and FT4 levels to confirm primary hypothyroidism, which is characterized by an elevated TSH with a low free T4.
- TSH and FT4 should be used for case detection in symptomatic patients.
- Low TSH with a low FT4 is consistent with central hypothyroidism, and further evaluation is needed. Some key points to consider in the diagnostic workup include:
- The serum TSH test is the primary screening test for thyroid dysfunction, and multiple tests should be done over a 3- to 6-month interval to confirm or rule out abnormal findings 1.
- Follow-up testing of serum T4 levels in persons with persistently abnormal TSH levels can differentiate between subclinical and overt thyroid dysfunction.
- The optimal screening interval for thyroid dysfunction is unknown, and more research is needed to determine the best approach. In terms of treatment, levothyroxine replacement therapy is typically recommended, starting at 1.6 mcg/kg/day for most adults, with lower initial doses for elderly patients or those with cardiovascular disease.
- TSH should be rechecked 6-8 weeks after starting treatment or changing doses, with the goal of normalizing TSH levels.
- Levothyroxine should be taken on an empty stomach, 30-60 minutes before breakfast or 3-4 hours after the last meal of the day, and separated from medications that can interfere with absorption.
From the FDA Drug Label
In adult patients with primary hypothyroidism, monitor serum TSH levels after an interval of 6 to 8 weeks after any change in dosage. In patients on a stable and appropriate replacement dosage, evaluate clinical and biochemical response every 6 to 12 months and whenever there is a change in the patient’s clinical status In patients with hypothyroidism, assess the adequacy of replacement therapy by measuring both serum TSH and total or free-T4. Monitor TSH and total or free-T4 in pediatric patients as follows: 2 and 4 weeks after the initiation of treatment, 2 weeks after any change in dosage, and then every 3 to 12 months thereafter following dosage stabilization until growth is completed The general aim of therapy is to normalize the serum TSH level
The diagnostic workup for hypothyroidism includes:
- Monitoring serum TSH levels
- Measuring total or free-T4 levels
- Evaluating clinical and biochemical response every 6 to 12 months
- Assessing serum TSH and free-T4 as soon as pregnancy is confirmed and during each trimester of pregnancy in pregnant patients 2
- Routine determinations of serum T4 and/or TSH is strongly advised in neonates 3
From the Research
Diagnostic Workup for Hypothyroidism
The diagnostic workup for hypothyroidism involves a combination of clinical evaluation, physical examination, and laboratory tests.
- The first line tests for hypothyroidism are analyses of the concentrations of free thyroxine (T4) and thyroid-stimulating hormone (TSH) in serum 4, 5, 6, 7.
- In primary hypothyroidism, the serum content of T4 is low and that of TSH is high 4, 5, 6.
- In central hypothyroidism, the serum content of T4 is low and that of TSH is generally low or normal 4, 5, 6.
- Subclinical hypothyroidism is characterized by a normal serum level of T4, an increased level of TSH, and the absence of clinical symptoms 4, 5, 6.
Laboratory Tests
The following laboratory tests are used to diagnose and monitor hypothyroidism:
- Thyroid-stimulating hormone (TSH) level: the preferred test for initial evaluation of suspected primary hypothyroidism 7.
- Free thyroxine (T4) level: used to further narrow the diagnosis if TSH is abnormal 7.
- Triiodothyronine (T3) level: obtained if TSH is undetectable and free T4 is normal 7.
- Free T4 and T3 concentrations: used to monitor central hypothyroidism, as TSH levels cannot be used 7.
Physical Examination
Physical examination is not reliable for diagnosing hypothyroidism, as no single sign can easily discriminate a euthyroid from a hypothyroid patient 8.