Increased free thyroxine in a euthyroid patient with thyroxine-binding globulin deficiency

Area of Science:

• Endocrinology
• Thyroid Hormone Metabolism
• Clinical Biochemistry

Background:

• Thyroxine-binding globulin (TBG) is the primary carrier protein for thyroxine (T4) in serum.
• Familial conditions affecting TBG levels can alter total T4 concentrations.
• Understanding free T4 dynamics is crucial for assessing thyroid status.

Observation:

• A euthyroid patient presented with familial low TBG and paradoxically elevated free thyroxine (FT4) levels.
• Total serum T4 was low, while triiodothyronine (T3) and thyroid-stimulating hormone (TSH) were normal.
• Thyroidal iodine uptake and T3/T4 tracer kinetics showed normal T3 and accelerated T4 clearance.

Findings:

• Markedly elevated free T4 concentration (9.2-12.1 ng/100 ml) was confirmed by dialysis.
• Normal T3 concentration (80 ng/100 ml) and normal T3 disappearance rate (t 1/2 = 2.1 days).
• Accelerated T4 disappearance (t 1/2 = 3.7 days) and a moderately elevated T4 degradation rate.

Implications:

• Suggests that T3 may play a more significant role in metabolic regulation than T4.
• Highlights the complexity of thyroid hormone homeostasis in individuals with genetic TBG abnormalities.
• Euthyroidism in this context may be maintained by a T4 disposal rate within the upper normal range.