Levothyroxine replacement doses are affected by gender and weight, but not age.

BACKGROUND: Body weight (BW) and age have been shown to affect the dose of levothyroxine (LT(4)) that results in normalization of serum thyroid-stimulating hormone (TSH) in hypothyroid patients. Our objective was to determine whether gender, menstrual status, and ideal BW (IBW) also affect the LT(4) dose required to achieve a serum TSH within the normal range. METHODS: We retrospectively reviewed the charts of patients being treated for primary hypothyroidism who had TSH values within a normal range. We selected patients aged 18-85 years who were taking LT(4) without any confounding medications, and who had no serious chronic conditions. Their LT(4) doses, referred to here as LT(4) dose requirements, based on both BW and IBW were documented. The relationship between gender, menstrual status, age, serum TSH concentrations, and the degree of overweight on LT(4) dose requirements were determined using multivariate analyses. RESULTS: Women were significantly more overweight than men (ratio of BW/IBW was 1.35 for women vs. 1.17 for men, p <0.0001). LT(4) requirements based on BW did not differ by gender when age was included in the model. However, when degree of overweight was also included, men required lower LT(4) doses than both premenopausal women (1.34 µg/kg vs. 1.51 µg/kg, p = 0.038) and menopausal women (1.34 µg/kg vs. 1.49 µg/kg, p = 0.023). When examining IBW using a model incorporating age, men also required lower LT(4) doses than both premenopausal women (1.64 µg/kg vs. 1.92 µg/kg, p = 0.0033) and menopausal women (1.64 µg/kg vs. 1.90 µg/kg, p = 0.0024). Serum TSH concentrations were not significantly different in any of the gender groups. There was no relationship between serum TSH and either age or BW. The initial serum TSH concentration was by design with the normal range, but the concentration within that range was not a significant predictor of the LT(4) replacement dose in any of the models. CONCLUSION: In contrast to previous studies suggesting that age affects LT(4) replacement requirements, we found that age-based differences in doses are secondary to differences in BW and gender. In addition, in contrast to prior studies showing that lean body mass, but not gender, affected LT(4) dose, we instead found a significant impact of gender. Gender-based differences in dose requirement only became apparent either when IBW was used to correct for the dose or when degree of overweight was included in the model. Gender differences in LT(4) dose requirement exist, but are masked unless gender-based differences in degree of overweight are also considered.

Graves' disease and thymic hyperplasia: the relationship of thymic volume to thyroid function.

BACKGROUND: Thymic hyperplasia is associated with Graves' disease, particularly in young patients. The degree of thymic transformation is minimal in most but not all patients. In the latter group radiological measurements of thyroid size and their change with treatment have rarely been reported. We present two patients with Graves' disease and relatively rapid resolution of thymic enlargement after successful treatment of their hyperthyroidism. SUMMARY: Three patients with thyrotoxicosis secondary to Graves' disease and marked thymic enlargement were seen at our institution during a 2-year period. On computed tomography (CT) studies their volumes were 67, 81, and 54 cm(3). Thymic hyperplasia in the setting of Graves' disease was the diagnosis of exclusion. Two of the patients returned for follow-up after successful treatment of thyrotoxicosis as requested. On repeat CT their thymic volumes had decreased by 72% and 78%, respectively. Two types of histological modifications of the thymus have been described in association with Graves' disease, namely, thymic parenchyma hyperplasia and medullary lymphoid hyperplasia. The mechanisms underlying thymic transformation in patients with Graves' hyperthyroidism are not completely elucidated, but autoimmune processes underlying Graves' disease are presumed to play a role. The clinical course of our patients is consistent with earlier literature, indicating that thymic enlargement may occur in conjunction with Graves' hyperthyroidism, and that it usually resolves as hyperthyroidism is treated, but there is little quantitative pre- and posttreatment of hyperthyroidism data. CONCLUSION: Although every patient must be individually considered, it appears that thymic hyperplasia can be diagnosed in most Graves' hyperthyroid patients by considering the clinical context and appropriate radiologic studies such as CT. Raising awareness of the association of thymic hyperplasia in patients with Graves' hyperthyroidism and its resolution with the reversibility of the hyperthyroid state should prevent unnecessary thymic evaluation and surgery with its attendant risks.

Hypothyroidism.

Hypothyroidism is one of the most common disorders encountered in an endocrine office practice. This article reviews the epidemiology, etiology, clinical presentation, diagnosis, and treatment of hypothyroidism. We emphasize some of the more recent issues, such as combination thyroid hormone therapy, management of hypothyroidism during pregnancy, and the management of subclinical hypothyroidism.