Effects of vitamin A administration on serum thyrotropin concentrations in healthy human subjects

Retinoids play an important role in the regulation of normal growth and development. Their biological action is mediated by a nuclear receptor that belongs to the steroid/thyroid hormone receptors superfamily. Retinoic acid has been shown to inhibit the secretion and synthesis of thyrotropin (TSH); however, little is known on the effects of retinoids on TSH secretion in normal human subjects. In the present study, we evaluated serum TSH concentration following both vitamin A (vit A) and the combined vit A and triiodothyronine (T(3)) administration. Basal and thyrotropin-releasing hormone (TRH)-stimulated TSH serum concentrations were measured in healthy young subjects in the following experimental conditions: (1) after 10 days of treatment with vit A orally administered as retinol at a dose of 50,000 IU/d; (2) after 10 days of oral placebo (PL) treatment; (3) after 1 hour from the administration of 40 mg T(3) at the end of 10 days of PL treatment; and (4) after 1 hour from the administration of 40 mg T(3) at the end of 10 days of vit A treatment. Serum TSH concentrations were also measured during vit A administration in healthy elderly subjects according to the following protocol: (1) after 10 days of treatment with PL; and (2) after 10 days of treatment with vit A at the same dose used for young subjects. In young subjects, basal serum TSH levels were found to be similar in the 4 different treatment conditions. In the same group of subjects, each of the 4 experimental conditions induced an increase in serum TSH, which rose from basal values of 1.80 +/- 0.31 to a peak of 11.92 +/- 1.75 microIU/mL (P <.001) during the PL treatment, from basal values of 1.81 +/- 0.22 to a peak of 10.81 +/- 1.00 microIU/mL (P <.001) during vit A treatment, from basal values of 1.72 +/- 0.28 to a peak of 9.92 +/- 1.10 microIU/mL (P <.001) during PL + T(3) treatment, and from basal values of 1.79 +/- 0.30 to a peak of 9.51 +/- 1.12 microIU/mL (P <.001) during vit A + T(3) treatment. The 2-way repeated measure analysis of variance revealed no significant differences among treatments. In old subjects, basal serum TSH levels were similar in the 2 experimental conditions and were not different from those observed in young subjects. In these subjects, serum TSH levels increased significantly in response to the TRH stimulus from basal values of 2.16 +/- 0.3 to a peak of 10.27 +/- 0.55 microIU/mL (P <.001) during PL treatment and from basal values of 2.10 +/- 0.51 to a peak of 7.82 +/- 1.4 microIU/mL (P <.001) during vit A treatment. No significant effects of treatment were found in this group of subjects on TRH-induced TSH levels; however, TSH responses were somewhat lower during vit A treatment with a difference close to statistical significance. These results suggest that TSH secretion is poorly affected by vit A administration in healthy human subjects; the data also indicate that any cooperation between T(3) and vit A is unlikely to occur in the regulation of TSH secretion.