OR14-06 Serum Thyroid Hormones Increase At The Onset Of Lactation And Promote Metabolic Pathways Required For Human Milk Synthesis

Disclosure: H. Rostom: None. X. Meng: None. R. Humphrey: None. A. Fry: None. T. Elajnaf: None. F. Hannan: None. An increase in mammary metabolism is critical for initiating lactation during postpartum days 1-4. We utilised clinical and cellular approaches to investigate whether thyroid hormones, which promote lactation in rodents, are involved in initiating human lactation and regulating mammary metabolism. We recruited n=35 healthy pregnant women intending to breastfeed following informed consent and measured serum thyroid hormones (TSH and free T4) at 36 weeks’ gestation and on postpartum days 1-4 (1). Serum TSH significantly increased at post-partum day 1 compared with 36 weeks’ gestation (2.5±0.3 vs. 1.6±0.1 mU/L, p=0.001). This early postpartum rise in TSH was followed by a significant increase in free T4 on post-partum days 3 and 4 (11.7±0.2 vs. 10.5±0.2pmol/L at 36 weeks’ gestation, p<0.01). We hypothesised that thyroid hormones increase mammary metabolism to promote breast milk production. Reverse transcription-quantitative PCR (RT-qPCR) of genes encoding deiodinase enzymes (DIO1-3), which regulate intracellular thyroid hormone activity, was undertaken in lactating human mammary epithelial cells (HMECs) isolated from breast milk. This showed that DIO1, which converts T4 to T3, was expressed in lactating HMECs, whereas DIO2 and DIO3 were not detected. Moreover, RT-qPCR of the thyroid hormone receptor alpha and beta (THRA and THRB) genes in lactating HMECs showed that THRB has ∼4-fold greater expression than THRA (p<0.05, n=4). THRB phosphorylates Akt, which is required for initiating lactation. Consistent with this, cultured HMECs stimulated with 10nM T3, the most potent thyroid hormone, for 15min showed an ∼2-fold increase in Akt phosphorylation (p<0.0001, n=4). Akt regulates oxidative phosphorylation, which we assessed by measuring oxygen consumption rate (OCR) and ATP synthesis. HMECs treated with 10nM T3 for ≤8hrs showed a 2-fold increase in OCR (p<0.05, n=4) without any change in cellular ATP. These findings suggested that T3 uncoupled mitochondrial respiration from ATP synthesis. In support of this, RT-qPCR of cultured HMECs stimulated with 10nM T3 showed increased expression of genes encoding uncoupling proteins 2 and 3 (≥2-fold, p<0.01, n=4), which also function to divert substrates away from mitochondrial catabolism; and >20-fold increased expression of PPARGC1A (p<0.0001, n=4), which promotes mitochondrial biogenesis. In summary, these findings demonstrate that serum TSH and free T4 are increased at the onset of lactation. Moreover, our cellular studies indicate that thyroid hormones promote mammary mitochondrial biogenesis and may divert substrates used for ATP generation towards the synthesis of breast milk. Reference: (1) Rostom, H. et al. Protocol for an observational study investigating hormones triggering the onset of sustained lactation: the INSIGHT study. BMJ Open 12, e062478. Presentation: Friday, June 16, 2023