Adenohypophyseal Function in Dogs with Primary Hypothyroidism and Nonthyroidal Illness

Abstract

Background: A recent study of dogs with induced primary hypothyroidism (PH) demonstrated that thyroid hormone deficiency leads to loss of thyrotropin (TSH) hypersecretion, hypersomatotropism, hypoprolactinemia, and pituitary enlargement with large vacuolated ''thyroid deficiency'' cells that double-stained for growth hormone (GH) and TSH, indicative of transdifferentiation of somatotropes to thyrosomatropes.Hypothesis: Similar functional changes in adenohypophyseal function occur in dogs with spontaneous PH as do in dogs with induced PH, but not in dogs with nonthyroidal illness (NTI).Animals: Fourteen dogs with spontaneous PH and 13 dogs with NTI. Methods: Adenohypophyseal function was investigated by combined intravenous administration of 4 hypophysiotropic releasing hormones (4RH test), followed by measurement of plasma concentrations of ACTH, GH, luteinizing hormone (LH), prolactin (PRL), and TSH. In the PH dogs this test was repeated after 4 and 12 weeks of thyroxine treatment.Results: In 6 PH dogs, the basal TSH concentration was within the reference range. In the PH dogs, the TSH concentrations did not increase with the 4RH test. However, TSH concentrations increased significantly in the NTI dogs. Basal and stimulated GH and PRL concentrations indicated reversible hypersomatotropism and hyperprolactinemia in the PH dogs, but not in the NTI dogs. Basal and stimulated LH and ACTH concentrations did not differ between groups.Conclusions and Clinical Importance: Dogs with spontaneous PH hypersecrete GH but have little or no TSH hypersecretion. Development of hyperprolactinemia (and possible galactorrhea) in dogs with PH seems to occur only in sexually intact bitches. In this group of dogs with NTI, basal and stimulated plasma adenohypophyseal hormone concentrations were not altered.

Effect of hypothyroidism on insulin sensitivity and glucose tolerance in dogs

“…The high basal serum GH and IGF-1 concentrations in hypothyroid dogs were consistent with findings of other studies [14][15][16] in this species. Serum GH concentration is reportedly high in hypothyroid humans with insulin resistance, 10 but other studies [64][65][66] in hypothyroid humans have revealed no change or a decrease in serum GH concentrations or response to GHRH.…”

“…For example, in hypothyroid insulin-resistant women, plasma concentrations of glucagon, epinephrine, cortisol, and GH were high before hormone replacement therapy, indicating that these hormones may contribute to the development of insulin resistance. 10 Growth hormone excess has been linked to insulin resistance in dogs, cats, and people, [11][12][13] and recent studies [14][15][16] have revealed high GH concentrations in hypothyroid dogs. Obesity, which is a common consequence of the reduced metabolism of hypothyroid dogs, may also negatively affect S I and glucose tolerance in dogs.…”

Hypothyroidism

“…and hypothyroidism and diabetes mellitus can occur together (Hess et al, 2003;Dixon et al, 1999). Increased concentrations of IGF-1 and GH have been documented in hypothyroid dogs and likely contribute to insulin resistance (Diaz-Espiñeira, 2009;Hofer-Inteeworn, 2012). Increased concentrations of IGF-1 and GH have been documented in hypothyroid dogs and likely contribute to insulin resistance (Diaz-Espiñeira, 2009;Hofer-Inteeworn, 2012).…”

Central Hypothyroidism in Miniature Schnauzers

“…Absence of an increase in plasma TSH concentration after TRH administration is not fully diagnostic for central hypothyroidism. Also in dogs with primary hypothyroidism, TRH administration does not result in a significant rise in plasma TSH concentration . In 5 of the dogs, the central nature of the hypothyroidism was confirmed by administering rhTSH on 3 consecutive days .…”

“…Differentiating primary and central hypothyroidism is problematic because TSH deficiency in central hypothyroidism results in atrophy of the thyroid gland, with resultant inadequate thyroxine secretion or radionuclide uptake on TSH stimulation testing and nuclear scintigraphy, respectively . The relatively low sensitivity of increased plasma TSH concentration in dogs with primary hypothyroidism contributes to the difficulty in the diagnosis of central hypothyroidism.…”