Thyroid hormones (TH) play an essential role in growth and differentiation, control of metabolism, and other physiological functions in virtually every human tissue. Thyroxine (T4), the major prohormone secreted by the thyroid gland, is largely converted into the active hormone 3,3′,5-triiodothyronine (T3) in peripheral tissues by specific enzymes called deiodinases [Deiodinase 1 (DIO1) and Deiodinase 2 (DIO2)] [
1]. Circulating TH levels are regulated via a negative feedback mechanism by the hypothalamus–pituitary–thyroid (HPT) axis. Thyrotropin-releasing hormone (TRH), produced by hypothalamus, stimulates anterior pituitary to release thyroid-stimulating hormone (TSH) that, binding TSH receptor (TSHR) expressed on the thyroid follicular cells, stimulates the TH synthesis. In turn, T4 and T3 negatively regulate TSH production [
2]. Hypothyroidism and hyperthyroidism are thyroid dysfunctions caused by an increase or decrease of TSH levels with or without decrease or increase of T4 levels, respectively. Hypothyroidism may be due to a number of causes including autoimmune disease, thyroid surgery, iodine deficiency, pituitary disorders and congenital diseases [
3]. Congenital hypothyroidism (CH) is a thyroid hormone deficiency present at birth for permanent or transient causes [
4]. Permanent CH can be primary, secondary (for TRH resistance or defects in TSH production) or peripheral [for defects in thyroid hormone transport genes, as solute carrier family 16 member 2 (
SLC16A2) and solute carrier family 16 member 10 (
SLC16A10), or resistance to TH]. Primary CH is related to conditions of dysgenesis or dyshormonogenesis, caused by alterations in genes involved in the growth and development of thyroid and in the TH biosynthesis and secretion, respectively [
4]. Genes associated with dysgenesis are thyroid transcription factor-1 (
TTF-1), thyroid transcription factor-2 (
TTF-2), paired box 8 (
PAX-8) and
TSHR, while dyshormonogenesis is associated with gene defects in thyroperoxidase (
TPO), thyroglobulin (
TG), dual oxidase 2 (
DUOX2), sodium-iodide symporter (
NIS), solute carrier family 5 member 5 (
SLC5A5), and pendrin (
SLC26A4) [
5,
6]. In addition to known mutations, the role of single nucleotide polymorphisms (SNPs) has been also investigated in CH. Different studies reported as polymorphic variants in
TPO,
TSHR and
TG genes can represent a genetic risk factor for dishormonogentic CH [
7,
8,
9,
10].
SNPs may be the basis of the evolutionary process, since they are heritable and modified by natural selection. During human evolution, interaction with factors such as climatic conditions, nutrients availability, and lifestyle habits have allowed the selection of favourable features [
11]. Natural selection has led to a gradual modification of genetic composition of population; individuals who were better adapted passed on their genes, including those conferring these benefits with an increasing frequency [
12]. In the process of adaptation, hormones and the endocrine system certainly have played a crucial role. Thanks to genome analysis of ancestral humans [
12,
13,
14,
15,
16,
17] it is possible to better understand human evolution and deepen the functionality of TH in our ancestors. In a previous work, we showed an evolutionary perspective for the rs225014 (p.Thr92Ala) variant of the
DIO2 gene from Neanderthals to Anatomically Modern Humans (AMH) [
18]. In fact, Neanderthals and Denisovans, with a diet characterised by low carbohydrate intake, displayed the alanine amino acid at position 92, associated with a reduced production of T3, while Modern Humans, with a high carbohydrate diet, exhibited threonine that is related to an increased production of T3 [
18]. These findings have shown that the
DIO2 rs225014 variant has been positively selected under particular living conditions and food habits. Considering this evidence, we investigated other variants in genes involved in the development of thyroid gland and TH biosynthesis, secretion and transport, comparing ancestral genomes of different human populations with contemporary humans. By studying a larger panel of genes, we will have the possibility to reconstruct with increasing accuracy the changes that TH and endocrine system have undergone in the course of human evolution.