Plasma thyrotropin, thyroxine, triiodothyronine, free thyroxine, free triiodothyronine, and cortisol levels in blind prepubertal boys

Chrono-Endocrinology in Clinical Practice: A Journey from Pathophysiological to Therapeutic Aspects

This review was aimed at collecting the knowledge on the pathophysiological and clinical aspects of endocrine rhythms and their implications in clinical practice, derived from the published literature and from some personal experiences on this topic. We chose to review, according to the PRISMA guidelines, the results of original and observational studies, reviews, meta-analyses and case reports published up to March 2024. Thus, after summarizing the general aspects of biological rhythms, we will describe the characteristics of several endocrine rhythms and the consequences of their disruption, paying particular attention to the implications in clinical practice. Rhythmic endocrine secretions, like other physiological rhythms, are genetically determined and regulated by a central hypothalamic CLOCK located in the suprachiasmatic nucleus, which links the timing of the rhythms to independent clocks, in a hierarchical organization for the regulation of physiology and behavior. However, some environmental factors, such as daily cycles of light/darkness, sleep/wake, and timing of food intake, may influence the rhythm characteristics. Endocrine rhythms are involved in important physiological processes and their disruption may cause several disorders and also cancer. Thus, it is very important to prevent disruptions of endocrine rhythms and to restore a previously altered rhythm by an early corrective chronotherapy.

Chronothyroidology: Chronobiological Aspects in Thyroid Function and Diseases

Chronobiology is the scientific discipline which considers biological phenomena in relation to time, which assumes itself biological identity. Many physiological processes are cyclically regulated by intrinsic clocks and many pathological events show a circadian time-related occurrence. Even the pituitary–thyroid axis is under the control of a central clock, and the hormones of the pituitary–thyroid axis exhibit circadian, ultradian and circannual rhythmicity. This review, after describing briefly the essential principles of chronobiology, will be focused on the results of personal experiences and of other studies on this issue, paying particular attention to those regarding the thyroid implications, appearing in the literature as reviews, metanalyses, original and observational studies until 28 February 2021 and acquired from two databases (Scopus and PubMed). The first input to biological rhythms is given by a central clock located in the suprachiasmatic nucleus (SCN), which dictates the timing from its hypothalamic site to satellite clocks that contribute in a hierarchical way to regulate the physiological rhythmicity. Disruption of the rhythmic organization can favor the onset of important disorders, including thyroid diseases. Several studies on the interrelationship between thyroid function and circadian rhythmicity demonstrated that thyroid dysfunctions may affect negatively circadian organization, disrupting TSH rhythm. Conversely, alterations of clock machinery may cause important perturbations at the cellular level, which may favor thyroid dysfunctions and also cancer.

Melatonin and the pituitary-thyroid axis status in blind adults: a possible resetting after puberty

OBJECTIVE

Increased levels of free thyroid hormones have been previously described in prepubertal blind subjects and have been thought to be a consequence of a partial target organ refractoriness due to the early and prolonged lack of light perception. The aim of this study was to clarify whether this abnormality is permanent or transient and the interrelationships between melatonin and thyroid hormone secretion.

MEASUREMENTS

Total and free thyroid hormones, TSH, thyroxine-binding globulin (TBG), reverse triiodothyronine (rT3) and melatonin were measured in plasma samples obtained at 0800 h (two hours after lights-on) in a group of 11 totally (group 1) and 16 partially (group 2) blind adult patients and in 10 age-matched healthy subjects.

RESULTS

Both totally and partially blind patients showed melatonin levels higher than in controls (330 +/- 106 pmol/l, group 1 and 361 +/- 159 pmol/l, group 2, respectively; controls: 53 +/- 12 pmol/l, P < 0.001 vs both groups), but fT4, fT3, T4, T3 TSH, rT3 and TBG concentrations showed no significant differences from controls.

CONCLUSIONS

A possible resetting of pituitary-thyroid axis regulation can occur in blindness after puberty; variations of melatonin secretion could play a role in this. The inhibitory effect of melatonin on thyroid gland function found in animals does not seem to occur in humans. Elevated melatonin levels, both in patients with total blindness and in those with light perception only, suggest that more complex mechanisms other than light signalling are involved in the changes of melatonin secretion in blindness.

Blindness influences the growth of institutionalized prepubertal subjects

To investigate if blindness influences the growth rate of young subjects we measured height and weight of 71 prepubertal blind subjects (33 females and 38 males) aged 7-10 yr, in Tanner stage one puberty, living at the Institute "Martuscelli" for young blind individuals in Naples, Italy. They were divided into two groups: group 1 consisting of 27 subjects (14 females and 13 males) with total blindness; group 2 consisting of 44 subjects (20 females and 24 males) having only a light perception. The distribution in quintiles of their height and weight was compared by X2 test for trend to that of an age-matched large population (7902 subjects: 3808 females and 4094 males) of primary school from the same district. A stature significantly lower than controls was evidenced in both group 1 (p = 0.0006) and in group 2 (p = 0.008). When the comparison is performed taking into account the sex of subjects, the girls of both group 1 (p = 0.0014) and group 2 (p = 0.0004) show a stature significantly lower than female controls, whereas the stature of boys of both groups did not differ from that of male controls. No statistically significant differences in weight distribution were found between controls and either group 1 and group 2. Our results suggest that total or partial blindness could influence negatively the growth of prepubertal subjects; in particular it can cause short stature or growth delay especially in female sex.