In hypothyroidism, what are the expected levels of total Thyroxine (T4), Thyroid Hormone Binding Ratio (THBR) (or Triiodothyronine (T3) uptake), and Thyroid-Stimulating Hormone (TSH)?

Expected Laboratory Values in Hypothyroidism

In hypothyroidism, one would expect the total T4 to be decreased, the Thyroid Hormone Binding Ratio (THBR) or T3 Uptake to be decreased, and the TSH level to be increased. The correct answer is C. 1, 2

Laboratory Profile in Hypothyroidism

Hypothyroidism presents with a characteristic pattern of thyroid function tests:

1. Total T4 (Thyroxine): Decreased

   • In hypothyroidism, the thyroid gland produces insufficient amounts of thyroid hormones
   • Total T4 levels fall below the normal range due to reduced thyroid gland output 2

2. THBR (Thyroid Hormone Binding Ratio) or T3 Uptake: Decreased

   • This test indirectly measures the availability of binding sites on thyroid-binding proteins
   • In hypothyroidism, there are more unoccupied binding sites on transport proteins
   • This results in decreased T3 uptake by the resin used in the test 2

3. TSH (Thyroid-Stimulating Hormone): Increased

   • The pituitary gland increases TSH production in response to low thyroid hormone levels
   • An elevated TSH is the most sensitive indicator of primary thyroid gland failure 1, 2

Diagnostic Considerations

The American College of Physicians recommends:

• Normal TSH range is approximately 0.4-4.5 mIU/L 1
• Diagnosis of hypothyroidism is based on TSH and free T4 levels 1
• Subclinical hypothyroidism: elevated TSH with normal free T4 levels
• Overt hypothyroidism: elevated TSH with low free T4 levels 1

Important Clinical Considerations

• Modern laboratory testing typically measures free T4 rather than total T4, as it better correlates with hormonal activity 1, 3
• Free T4 is the metabolically active form (only 0.02% of total T4) 3
• In central (secondary) hypothyroidism due to pituitary dysfunction, TSH may be inappropriately normal or low despite low T4 levels 1
• T3/T4 ratio may correlate with clinical response to treatment, with a lower ratio associated with persistent symptoms despite normal TSH 4

Common Pitfalls

• Failing to differentiate between primary and central hypothyroidism
• Not considering non-thyroidal illness which can affect thyroid function tests
• Overlooking the possibility of pituitary dysfunction affecting both TSH and other hormones 1
• Relying solely on TSH without measuring free T4 to differentiate between subclinical and overt hypothyroidism 1

In summary, the laboratory pattern in hypothyroidism shows decreased total T4, decreased THBR (T3 uptake), and increased TSH, corresponding to answer choice C.