Does thyroid replacement therapy affect pulmonary function tests in patients with subclinical hypothyroidism?

Subclinical hypothyroidism is an elevation in serum thyroid-stimulating hormone (TSH) while having normal serum free thyroxine (FT4) and triiodothyronine (FT3) levels. The purpose of this prospective observational study was to evaluate the pulmonary function of patients diagnosed with subclinical hypothyroidism, both before and after treatment with thyroid hormone.

This study took place at the Medical Faculty, Celal Bayar University, Manisa, Turkey between February 2003 and June 2004. Thirty-eight patients (37 females, one male) with subclinical hypothyroidism between 20 and 65 years of age were included in the study. Most were mildly obese. Arterial blood gases and pulmonary function tests were performed before treatment with thyroid hormone, and afterwards, the TSH value reached the normal range (indicating euthyroidism).

Oxygen saturation, but not partial oxygen pressure or partial carbon dioxide pressure, was statistically, but not clinically significantly higher after treatment with thyroid hormone (p=0.01). Pulmonary function tests were not significantly different before and after treatment with thyroid hormone.

In our subclinical hypothyroidism patients, pulmonary function tests were normal and did not significantly change with thyroid hormone replacement. The advantages of thyroid hormone replacement therapy, at least regarding respiratory function, seem to be clearly present in patients with overt, clinical hypothyroidism but not in patients with subclinical hypothyroidism.