Introduction: Permanent hypothyroidism is a common side effect of thyroidectomy. Thyroxine binding globulin (TBG) deficiency is characterized with low total thyroid hormones (TH) in the presence of normal free THs and thyroid stimulating hormone (TSH).
Case report: A previously healthy 37-year-old man underwent annual health check-up. Thyroid US revealed a 24×14×19 mm isoechoic solid nodule in the left lobe. TSH was 1.4 (0.274.2 μIU/ml). Fine needle aspiration (FNA) revealed Bethesda Category IV (follicular neoplasm or suspicious for follicular neoplasm) and thyroidectomy was recommended. He admitted to endocrinology outpatient clinic (EOC) for preoperative assessment. Thyroid antibodies, calcitonin, parathyroid hormone, calcium and phosphorus were normal. Left lobectomy was performed. Surgical pathology revealed a follicular adenoma. General surgery outpatient clinic referred patient to EOC 1.5 months later with low total TH levels (TSH: 2.12, total triiodothyronine (TT3): 0.41 (0.82 ng/ml), total thyroxine (TT4): 3.41 (5.114.10 μg/dl)). Laboratory tests were repeated: TSH: 1.9, free T4: 1.57 (0.931.7 ng/dl), free T3: 2.29 (1.84.6 pg/ml) were normal while TT3: 0.35 and TT4: 2.81 were low. Low total THs in the presence of normal TSH and free THs suggested TBG deficiency (TBGD). TBG was 3.54 (1431 μg/ml). In the absence of secondary disorders (malnutrition, liver or renal disease, drugs) causing low TBG levels were, he was diagnosed as having inherited TBGD (iTBGD). He was informed about iTBGD but he declined any genetic analysis.
Conclusions: TBGD is a nonharmful condition that is either acquired (aTBGD) due to lack of protein supply or synthesis, loss of urinary protein or X linked inherited TBGD (iTBGD). Complications are those stemming from the primary disorders and erroneously administered L-thyroxine treatment. In complete iTBGD males have no detectable TBG while in partial iTBGD, males have some measurable TBG. Evaluation of thyroid function only with total THs may lead to a misdiagnosis of hypothyroidism and unnecessary treatment.