Rationalization of the Irrational Neuropathologic Basis of Hypothyroidism-Olfaction Disorders Paradox: Experimental Study

Abstract

Objective: Hypothyroidism is defined as an underactive thyroid gland and one of the reasons for inadequate stimulation of thyroid is dysfunction of the hormone regulating brain centers. Olfaction disorders have been considered as a problem in hypothyroidism. It has been hypothesized that olfaction disorders reduce olfactory stimulation and diminished olfactory stimulus may trigger hypothyroidism. In this study, an examination was made of the thyroid hormone levels, histologic features of thyroid glands, and vagal nerve network degradation in an experimental animal model of olfactory bulbectomy (OBX).

Methods: A total of 25 rats were divided into control (n = 5), SHAM (n = 5), and OBX (n = 15) groups and were followed up for 8 weeks. Thyroid hormone levels were measured before (1 time), during the experiment (1 time/month) and the animals were decapitated. The olfactory bulbs, dorsal motor nucleus of the vagal nerves, and thyroid gland sections were stained with hematoxylin-eosin and tunnel dye to determine OBX-related damage. Specimens were analyzed stereologically to evaluate neuron density of the vagal nucleus and hormone-filled total follicle volume (TFV) per cubic centimeter, and these were statistically compared with thyroid hormone levels.

Results: The mean degenerated neuron density of the vagal nucleus was 21 ± 8/mm³. TFV and triiodothyronine (T₃)-thyroxine (T₄) levels were measured as TFV, (312 ± 91) × 10⁶μm³/cm³; T₃, 105 μg/dl; T₄, 1.89 μg/dl in control (group I). Mean degenerated neuron density, 56 ± 12/mm³; TFV, (284 ± 69) × 10⁶μm³/cm³; T₃, 103 μg/dl; T₄, 1.85 μg/dl in SHAM (group II). Mean degenerated neuron density, 235 ± 64/mm³; TFV, (193 ± 34) × 10⁶μm³/cm³; T₃, 86 μg/dl; T₄, 1.37 μg/dl in the OBX group (group III). The TFV were significantly diminished because of apoptotic degradation in olfactory bulbs and thyroid gland with decreased T₃- T₄ levels with increased thyroid-stimulating hormone levels in OBX-applied animals of subarachnoid hemorrhage (P < 0.005).

Conclusions: The results suggested that diminished hormone secretion as a result of thyroid gland degradation results in both olfaction loss and vagal complex degeneration in OBX animals, contrary to the common belief that anosmia results from hypothyroidism.

Keywords: Hypothyroidism; Olfaction disorders; Olfactory bulbectomy; Vagal complex.